protected function getErrorMessage($http_status_code,
                                     $http_result,
                                     $msg_prefix = "Cloud Storage Error:") {
    if ($this->tryParseCloudStorageErrorMessage($http_result,
                                                $code,
                                                $message)) {
      return sprintf("%s %s (%s)", $msg_prefix, $message, $code);
    } else {
      return sprintf("%s %s",
                     $msg_prefix,
                     HttpResponse::getStatusMessage($http_status_code));
    }
  }