code: func replyError(c context.Context, rw http.ResponseWriter, err error) {
  	logging.Errorf(c, "Error while processing PubSub notification - %s", err)
  	if transient.Tag.In(err) {
  		http.Error(rw, err.Error(), http.StatusInternalServerError)
  	} else {
  		http.Error(rw, err.Error(), http.StatusBadRequest)
  	}
  }
docs: // replyError sends HTTP 500 on transient errors, HTTP 400 on fatal ones.