code: func replyError(c context.Context, rw http.ResponseWriter, err error) {
  	if transient.Tag.In(err) {
  		rw.WriteHeader(http.StatusInternalServerError)
  	} else {
  		rw.WriteHeader(http.StatusBadRequest)
  	}
  	templates.MustRender(c, rw, "pages/error.html", templates.Args{
  		"Error": err.Error(),
  	})
  }
docs: // replyError sends HTML error page with status 500 on transient errors or 400
  // on fatal ones.