fixed ErrorResponse call in API::setupPublic()

- removed empty constructors in updated endpoints
- added missing keys to Error class (unauthorized, forbidden)

lib/API/Error.php

@@ -6,6 +6,8 @@ if(!defined('ABSPATH')) exit;
 final class Error {
   const UNKNOWN = 'unknown';
   const BAD_REQUEST = 'bad_request';
+  const UNAUTHORIZED = 'unauthorized';
+  const FORBIDDEN = 'forbidden';
 
   private function __construct() {