code: func IsAuthError(err error) bool {
  	if e, ok := err.(interface {
  		AuthError() bool
  	}); ok {
  		return e.AuthError()
  	}
  	if parent := parentOf(err); parent != nil {
  		return IsAuthError(parent)
  	}
  	return false
  }
docs: // IsAuthError indicates the client provided a request with invalid or no
  // authentication credentials when credentials are required (e.g. HTTP 401).