/**
  * Only validates the credentials, don't actually login.
  *
  * @param User $User
  * @return string Error message, empty if credentials are valid
  */
 public function loginTest(\model\User $User)
 {
     if (empty($_POST[self::$name])) {
         return "Username is missing";
     } else {
         if (empty($_POST[self::$password])) {
             return "Password is missing";
         } else {
             if (!self::$LoginModel->checkCredential($User)) {
                 return "Wrong name or password";
             } else {
                 return '';
             }
         }
     }
 }