code: func (s *FileStorage) SiteExists(domain string) (bool, error) {
  	_, err := os.Stat(s.siteCertFile(domain))
  	if os.IsNotExist(err) {
  		return false, nil
  	} else if err != nil {
  		return false, err
  	}
  
  	_, err = os.Stat(s.siteKeyFile(domain))
  	if err != nil {
  		return false, err
  	}
  	return true, nil
  }
docs: // SiteExists implements Storage.SiteExists by checking for the presence of
  // cert and key files.