code: func replyWithError(w http.ResponseWriter, status, message string, args ...interface{}) error {
  	return replyWithJSON(w, map[string]string{
  		"status":        status,
  		"error_message": fmt.Sprintf(message, args...),
  	})
  }
docs: // replyWithError sends StatusOK with JSON body containing an error.
  //
  // Due to Cloud Endpoints limitations, legacy API used StatusOK for some not-OK
  // responses and communicated the actual error through 'status' response field.