code: func NewFailOnError(authTokenHandlers ...authenticator.Token) authenticator.Token {
  	if len(authTokenHandlers) == 1 {
  		return authTokenHandlers[0]
  	}
  	return &unionAuthTokenHandler{Handlers: authTokenHandlers, FailOnError: true}
  }
docs: // NewFailOnError returns a token authenticator that validates credentials using a chain of authenticator.Token objects.
  // The first error short-circuits the chain.