opts.go
func Errorf(f func(string, ...interface{})) Option {
	return func(s *Sentinel) error {
		s.errf = f
		return nil
	}
}
func Errorf(f func(string, ...interface{})) Option {
	return func(s *Sentinel) error {
		s.errf = f
		return nil
	}
}
// Errorf is a sentinel option to set a error logger.