fixed ErrorResponse call in API::setupPublic()

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

lib/API/Endpoints/Settings.php

@@ -7,9 +7,6 @@ use \MailPoet\Models\Setting;
 if(!defined('ABSPATH')) exit;
 
 class Settings extends APIEndpoint {
-  function __construct() {
-  }
-
   function get() {
     return $this->successResponse(Setting::getAll());
   }