code: func IsNotFoundError(err error) (bool, error) {
  	e, ok := errs.Cause(err).(NotFoundError)
  	if !ok {
  		return false, nil
  	}
  	return true, e
  }
docs: // IsNotFoundError returns true if the cause of the given error can be
  // converted to an NotFoundError, which is returned as the second result.