code: func (c *requestContext) fail(code int, msg string, args ...interface{}) {
  	body := fmt.Sprintf(msg, args...)
  	logging.Errorf(c.Context, "HTTP %d: %s", code, body)
  	http.Error(c.Writer, body, code)
  }
docs: // fail writes error message to the log and the response and sets status code.