General: Introduce WP_Error::has_errors() method and use it where appropriate.

Props robdxw, DrewAPicture, SergeyBiryukov.
Fixes #42742.

git-svn-id: https://develop.svn.wordpress.org/trunk@42761 602fd350-edb4-49c9-b593-d223f7449a82

src/wp-includes/user.php

@@ -2469,7 +2469,7 @@ function register_new_user( $user_login, $user_email ) {
 	 */
 	$errors = apply_filters( 'registration_errors', $errors, $sanitized_user_login, $user_email );
 
-	if ( $errors->get_error_code() ) {
+	if ( $errors->has_errors() ) {
 		return $errors;
 	}