REST API: Normalize WP_REST_Server::dispatch() to return a response object.

Previously, the `rest_pre_dispatch` filter could be used to return a `WP_Error` instance. This would cause a fatal error for `rest_post_dispath` 
filters that were rightly expecting a `WP_REST_Response` object to be passed instead.

Props DaveFX, felipeelia.
Fixes #56566.

Built from https://develop.svn.wordpress.org/trunk@55361


git-svn-id: http://core.svn.wordpress.org/trunk@54894 1a063a9b-81f0-0310-95a4-ce76da25c4cd

wp-includes/rest-api/class-wp-rest-server.php

@@ -984,6 +984,15 @@ class WP_REST_Server {
 		$result = apply_filters( 'rest_pre_dispatch', null, $this, $request );
 
 		if ( ! empty( $result ) ) {
+
+			// Normalize to either WP_Error or WP_REST_Response...
+			$result = rest_ensure_response( $result );
+
+			// ...then convert WP_Error across.
+			if ( is_wp_error( $result ) ) {
+				$result = $this->error_to_response( $result );
+			}
+
 			return $result;
 		}