code: func noAccessErr(c context.Context, prefix string) error {
  	return status.Errorf(
  		codes.PermissionDenied, "prefix %q doesn't exist or %q is not allowed to see it",
  		prefix, auth.CurrentIdentity(c))
  }
docs: // noAccessErr produces a grpc error saying that the given prefix doesn't
  // exist or the caller has no access to it. This is generic error message that
  // should not give away prefix presence to non-readers.