code: func (s *FileStorage) readFile(file string) ([]byte, error) {
  	b, err := ioutil.ReadFile(file)
  	if os.IsNotExist(err) {
  		return nil, ErrNotExist(err)
  	}
  	return b, err
  }
docs: // readFile abstracts a simple ioutil.ReadFile, making sure to return an
  // ErrNotExist instance when the file is not found.