user/error.go
func IsExist(err error) bool {
	if err == ErrUserExist || err == ErrGroupExist {
		return true
	}
	return false
}
func IsExist(err error) bool {
	if err == ErrUserExist || err == ErrGroupExist {
		return true
	}
	return false
}
// IsExist returns whether the error is known to report that an user or group
// already exists. It is satisfied by ErrUserExist and ErrGroupExist.