code: func TagGRPC(c context.Context, err error) error {
  	loggedIn := auth.CurrentIdentity(c) != identity.AnonymousIdentity
  	code := grpcutil.Code(err)
  	if code == codes.NotFound || code == codes.PermissionDenied {
  		// Mask the errors, so they look the same.
  		if loggedIn {
  			return errors.Reason("not found").Tag(grpcutil.NotFoundTag).Err()
  		} else {
  			return errors.Reason("not logged in").Tag(grpcutil.UnauthenticatedTag).Err()
  		}
  	}
  	return grpcutil.ToGRPCErr(err)
  }
docs: // TagGRPC annotates some gRPC with Milo specific semantics, specifically:
  // * Marks the error as Unauthorized if the user is not logged in,
  // and the underlying error was a 403 or 404.
  // * Otherwise, tag the error with the original error code.