diff --git a/src/wp-admin/includes/ajax-actions.php b/src/wp-admin/includes/ajax-actions.php
index 5e86d792dd..dc965d63cc 100644
--- a/src/wp-admin/includes/ajax-actions.php
+++ b/src/wp-admin/includes/ajax-actions.php
@@ -3345,16 +3345,25 @@ function wp_ajax_install_theme() {
 		wp_send_json_error( $status );
 	}
 
-	$upgrader = new Theme_Upgrader( new Automatic_Upgrader_Skin() );
+	$skin     = new WP_Ajax_Upgrader_Skin();
+	$upgrader = new Theme_Upgrader( $skin );
 	$result   = $upgrader->install( $api->download_link );
 
 	if ( defined( 'WP_DEBUG' ) && WP_DEBUG ) {
-		$status['debug'] = $upgrader->skin->get_upgrade_messages();
+		$status['debug'] = $skin->get_upgrade_messages();
 	}
 
 	if ( is_wp_error( $result ) ) {
+		$status['errorCode']    = $result->get_error_code();
 		$status['errorMessage'] = $result->get_error_message();
 		wp_send_json_error( $status );
+	} elseif ( is_wp_error( $skin->result ) ) {
+		$status['errorCode']    = $skin->result->get_error_code();
+		$status['errorMessage'] = $skin->result->get_error_message();
+		wp_send_json_error( $status );
+	} elseif ( $skin->get_errors()->get_error_code() ) {
+		$status['errorMessage'] = $skin->get_error_messages();
+		wp_send_json_error( $status );
 	} elseif ( is_null( $result ) ) {
 		global $wp_filesystem;
 
@@ -3437,14 +3446,22 @@ function wp_ajax_update_theme() {
 		wp_update_themes();
 	}
 
-	$upgrader = new Theme_Upgrader( new Automatic_Upgrader_Skin() );
+	$skin     = new WP_Ajax_Upgrader_Skin();
+	$upgrader = new Theme_Upgrader( $skin );
 	$result   = $upgrader->bulk_upgrade( array( $stylesheet ) );
 
 	if ( defined( 'WP_DEBUG' ) && WP_DEBUG ) {
-		$status['debug'] = $upgrader->skin->get_upgrade_messages();
+		$status['debug'] = $skin->get_upgrade_messages();
 	}
 
-	if ( is_array( $result ) && ! empty( $result[ $stylesheet ] ) ) {
+	if ( is_wp_error( $skin->result ) ) {
+		$status['errorCode']    = $skin->result->get_error_code();
+		$status['errorMessage'] = $skin->result->get_error_message();
+		wp_send_json_error( $status );
+	} elseif ( $skin->get_errors()->get_error_code() ) {
+		$status['errorMessage'] = $skin->get_error_messages();
+		wp_send_json_error( $status );
+	} elseif ( is_array( $result ) && ! empty( $result[ $stylesheet ] ) ) {
 
 		// Theme is already at the latest version.
 		if ( true === $result[ $stylesheet ] ) {
@@ -3458,10 +3475,6 @@ function wp_ajax_update_theme() {
 		}
 
 		wp_send_json_success( $status );
-	} elseif ( is_wp_error( $upgrader->skin->result ) ) {
-		$status['errorCode']    = $upgrader->skin->result->get_error_code();
-		$status['errorMessage'] = $upgrader->skin->result->get_error_message();
-		wp_send_json_error( $status );
 	} elseif ( false === $result ) {
 		global $wp_filesystem;
 
@@ -3594,16 +3607,25 @@ function wp_ajax_install_plugin() {
 
 	$status['pluginName'] = $api->name;
 
-	$upgrader = new Plugin_Upgrader( new Automatic_Upgrader_Skin() );
+	$skin     = new WP_Ajax_Upgrader_Skin();
+	$upgrader = new Plugin_Upgrader( $skin );
 	$result   = $upgrader->install( $api->download_link );
 
 	if ( defined( 'WP_DEBUG' ) && WP_DEBUG ) {
-		$status['debug'] = $upgrader->skin->get_upgrade_messages();
+		$status['debug'] = $skin->get_upgrade_messages();
 	}
 
 	if ( is_wp_error( $result ) ) {
+		$status['errorCode']    = $result->get_error_code();
 		$status['errorMessage'] = $result->get_error_message();
 		wp_send_json_error( $status );
+	} elseif ( is_wp_error( $skin->result ) ) {
+		$status['errorCode']    = $skin->result->get_error_code();
+		$status['errorMessage'] = $skin->result->get_error_message();
+		wp_send_json_error( $status );
+	} elseif ( $skin->get_errors()->get_error_code() ) {
+		$status['errorMessage'] = $skin->get_error_messages();
+		wp_send_json_error( $status );
 	} elseif ( is_null( $result ) ) {
 		global $wp_filesystem;
 
@@ -3680,19 +3702,22 @@ function wp_ajax_update_plugin() {
 
 	wp_update_plugins();
 
-	$skin     = new Automatic_Upgrader_Skin();
+	$skin     = new WP_Ajax_Upgrader_Skin();
 	$upgrader = new Plugin_Upgrader( $skin );
 	$result   = $upgrader->bulk_upgrade( array( $plugin ) );
 
 	if ( defined( 'WP_DEBUG' ) && WP_DEBUG ) {
-		$status['debug'] = $upgrader->skin->get_upgrade_messages();
+		$status['debug'] = $skin->get_upgrade_messages();
 	}
 
-	if ( is_array( $result ) && empty( $result[ $plugin ] ) && is_wp_error( $skin->result ) ) {
-		$result = $skin->result;
-	}
-
-	if ( is_array( $result ) && ! empty( $result[ $plugin ] ) ) {
+	if ( is_wp_error( $skin->result ) ) {
+		$status['errorCode']    = $skin->result->get_error_code();
+		$status['errorMessage'] = $skin->result->get_error_message();
+		wp_send_json_error( $status );
+	} elseif ( $skin->get_errors()->get_error_code() ) {
+		$status['errorMessage'] = $skin->get_error_messages();
+		wp_send_json_error( $status );
+	} elseif ( is_array( $result ) && ! empty( $result[ $plugin ] ) ) {
 		$plugin_update_data = current( $result );
 
 		/*
@@ -3716,9 +3741,6 @@ function wp_ajax_update_plugin() {
 			$status['newVersion'] = sprintf( __( 'Version %s' ), $plugin_data['Version'] );
 		}
 		wp_send_json_success( $status );
-	} elseif ( is_wp_error( $result ) ) {
-		$status['errorMessage'] = $result->get_error_message();
-		wp_send_json_error( $status );
 	} elseif ( false === $result ) {
 		global $wp_filesystem;
 
diff --git a/src/wp-admin/includes/class-wp-ajax-upgrader-skin.php b/src/wp-admin/includes/class-wp-ajax-upgrader-skin.php
new file mode 100644
index 0000000000..8698e20445
--- /dev/null
+++ b/src/wp-admin/includes/class-wp-ajax-upgrader-skin.php
@@ -0,0 +1,132 @@
+<?php
+/**
+ * Upgrader API: WP_Ajax_Upgrader_Skin class
+ *
+ * @package WordPress
+ * @subpackage Upgrader
+ * @since 4.6.0
+ */
+
+/**
+ * Upgrader Skin for Ajax WordPress upgrades.
+ *
+ * This skin is designed to be used for Ajax updates.
+ *
+ * @since 4.6.0
+ *
+ * @see Automatic_Upgrader_Skin
+ */
+class WP_Ajax_Upgrader_Skin extends Automatic_Upgrader_Skin {
+
+	/**
+	 * Holds the WP_Error object.
+	 *
+	 * @since 4.6.0
+	 * @access protected
+	 * @var null|WP_Error
+	 */
+	protected $errors = null;
+
+	/**
+	 * Constructor.
+	 *
+	 * @since 4.6.0
+	 * @access public
+	 *
+	 * @param array $args Options for the upgrader, see WP_Upgrader_Skin::__construct().
+	 */
+	public function __construct( $args = array() ) {
+		parent::__construct( $args );
+
+		$this->errors = new WP_Error();
+	}
+
+	/**
+	 * Retrieves the list of errors.
+	 *
+	 * @since 4.6.0
+	 * @access public
+	 *
+	 * @return WP_Error Errors during an upgrade.
+	 */
+	public function get_errors() {
+		return $this->errors;
+	}
+
+	/**
+	 * Retrieves a string for error messages.
+	 *
+	 * @since 4.6.0
+	 * @access public
+	 *
+	 * @return string Error messages during an upgrade.
+	 */
+	public function get_error_messages() {
+		$messages = array();
+
+		foreach ( $this->errors->get_error_codes() as $error_code ) {
+			if ( $this->errors->get_error_data( $error_code ) && is_string( $this->errors->get_error_data( $error_code ) ) ) {
+				$messages[] = $this->errors->get_error_message( $error_code ) . ' ' . esc_html( strip_tags( $this->errors->get_error_data( $error_code ) ) );
+			} else {
+				$messages[] = $this->errors->get_error_message( $error_code );
+			}
+		}
+
+		return implode( ', ', $messages );
+	}
+
+	/**
+	 * Stores a log entry for an error.
+	 *
+	 * @since 4.6.0
+	 * @access public
+	 *
+	 * @param string|WP_Error $errors Errors.
+	 */
+	public function error( $errors ) {
+		if ( is_string( $errors ) ) {
+			$string = $errors;
+			if ( ! empty( $this->upgrader->strings[ $string ] ) ) {
+				$string = $this->upgrader->strings[ $string ];
+			}
+
+			if ( false !== strpos( $string, '%' ) ) {
+				$args = func_get_args();
+				$args = array_splice( $args, 1 );
+				if ( ! empty( $args ) ) {
+					$string = vsprintf( $string, $args );
+				}
+			}
+
+			// Count existing errors to generate an unique error code.
+			$errors_count = count( $errors->get_error_codes() );
+			$this->errors->add( 'unknown_upgrade_error_' . $errors_count + 1 , $string );
+		} elseif ( is_wp_error( $errors ) ) {
+			foreach ( $errors->get_error_codes() as $error_code ) {
+				$this->errors->add( $error_code, $errors->get_error_message( $error_code ), $errors->get_error_data( $error_code ) );
+			}
+		}
+
+		$args = func_get_args();
+		call_user_func_array( array( $this, 'parent::error' ), $args );
+	}
+
+	/**
+	 * Stores a log entry.
+	 *
+	 * @since 4.6.0
+	 * @access public
+	 *
+	 * @param string|array|WP_Error $data Log entry data.
+	 */
+	public function feedback( $data ) {
+		if ( is_wp_error( $data ) ) {
+			foreach ( $data->get_error_codes() as $error_code ) {
+				$this->errors->add( $error_code, $data->get_error_message( $error_code ), $data->get_error_data( $error_code ) );
+			}
+		}
+
+		$args = func_get_args();
+		call_user_func_array( array( $this, 'parent::feedback' ), $args );
+	}
+}
diff --git a/src/wp-admin/includes/class-wp-upgrader-skins.php b/src/wp-admin/includes/class-wp-upgrader-skins.php
index 6b8fbdb735..8b2789da0c 100644
--- a/src/wp-admin/includes/class-wp-upgrader-skins.php
+++ b/src/wp-admin/includes/class-wp-upgrader-skins.php
@@ -36,3 +36,6 @@ require_once ABSPATH . 'wp-admin/includes/class-language-pack-upgrader-skin.php'
 
 /** Automatic_Upgrader_Skin class */
 require_once ABSPATH . 'wp-admin/includes/class-automatic-upgrader-skin.php';
+
+/** WP_Ajax_Upgrader_Skin class */
+require_once ABSPATH . 'wp-admin/includes/class-wp-ajax-upgrader-skin.php';
diff --git a/src/wp-admin/includes/class-wp-upgrader.php b/src/wp-admin/includes/class-wp-upgrader.php
index d5e10c4c64..2276152cc4 100644
--- a/src/wp-admin/includes/class-wp-upgrader.php
+++ b/src/wp-admin/includes/class-wp-upgrader.php
@@ -39,6 +39,9 @@ require_once ABSPATH . 'wp-admin/includes/class-language-pack-upgrader-skin.php'
 /** Automatic_Upgrader_Skin class */
 require_once ABSPATH . 'wp-admin/includes/class-automatic-upgrader-skin.php';
 
+/** WP_Ajax_Upgrader_Skin class */
+require_once ABSPATH . 'wp-admin/includes/class-wp-ajax-upgrader-skin.php';
+
 /**
  * Core class used for upgrading/installing a local set of files via
  * the Filesystem Abstraction classes from a Zip file.
diff --git a/src/wp-admin/js/updates.js b/src/wp-admin/js/updates.js
index d4274fef6a..5df8ded165 100644
--- a/src/wp-admin/js/updates.js
+++ b/src/wp-admin/js/updates.js
@@ -244,7 +244,7 @@
 	 * @param {string=} response.errorCode Optional. Error code for an error that occurred.
 	 */
 	wp.updates.ajaxAlways = function( response ) {
-		if ( ! response.errorCode && 'unable_to_connect_to_filesystem' !== response.errorCode ) {
+		if ( ! response.errorCode || 'unable_to_connect_to_filesystem' !== response.errorCode ) {
 			wp.updates.ajaxLocked = false;
 			wp.updates.queueChecker();
 		}
@@ -1533,7 +1533,7 @@
 	 * @returns {boolean} Whether there is an error that needs to be handled or not.
 	 */
 	wp.updates.maybeHandleCredentialError = function( response, action ) {
-		if ( response.errorCode && 'unable_to_connect_to_filesystem' === response.errorCode ) {
+		if ( wp.updates.shouldRequestFilesystemCredentials && response.errorCode && 'unable_to_connect_to_filesystem' === response.errorCode ) {
 			wp.updates.credentialError( response, action );
 			return true;
 		}