Filesystem API: Prevent fatal error in move_dir().

Correctly instantiate `WP_Error()` within `move_dir()` to prevent a fatal error when unable to delete an existing directory that is intended to be replaced.

Follow-up to [55204], [55219], [55220], [55223].

Props swissspidy, costdev, afragen.
Fixes #57375.



git-svn-id: https://develop.svn.wordpress.org/trunk@55226 602fd350-edb4-49c9-b593-d223f7449a82

src/wp-admin/includes/file.php

@@ -1979,7 +1979,7 @@ function move_dir( $from, $to, $overwrite = false ) {
 			return new WP_Error( 'destination_already_exists_move_dir', __( 'The destination folder already exists.' ), $to );
 		} elseif ( ! $wp_filesystem->delete( $to, true ) ) {
 			// Can't overwrite if the destination couldn't be deleted.
-			return WP_Error( 'destination_not_deleted_move_dir', __( 'The destination directory already exists and could not be removed.' ) );
+			return new WP_Error( 'destination_not_deleted_move_dir', __( 'The destination directory already exists and could not be removed.' ) );
 		}
 	}