code: func respondWithError(w http.ResponseWriter, err error) {
  	switch v := err.(type) {
  	case ct.ValidationError:
  		httphelper.ValidationError(w, v.Field, v.Message)
  	default:
  		if err == ErrNotFound {
  			w.WriteHeader(404)
  			return
  		}
  		httphelper.Error(w, err)
  	}
  }
docs: // NOTE: this is temporary until httphelper supports custom errors