Bootstrap/Load: Fix fatal error when passing a WP_Error to wp_die().

This was introduced in [44466]. Also, this changeset adds tests for `_wp_die_process_input()` so that this never happens again.

Props dd32.
See #45933.


git-svn-id: https://develop.svn.wordpress.org/trunk@44690 602fd350-edb4-49c9-b593-d223f7449a82

src/wp-includes/functions.php

@@ -3374,7 +3374,7 @@ function _wp_die_process_input( $message, $title = '', $args = array() ) {
 					$errors[] = array(
 						'code'    => $error_code,
 						'message' => $error_message,
-						'data'    => $error->get_error_data( $error_code ),
+						'data'    => $message->get_error_data( $error_code ),
 					);
 				}
 			}