code: func isBadTokenError(err error) bool {
  	if rerr, _ := err.(*oauth2.RetrieveError); rerr != nil {
  		return rerr.Response.StatusCode == 400 || rerr.Response.StatusCode == 401
  	}
  	return false
  }
docs: // isBadTokenError sniffs out HTTP 400/401 from token source errors.