code: func loadCheckpoint(checkpointFile string) (string, error) {
  	content, err := ioutil.ReadFile(checkpointFile)
  	if err != nil {
  		if os.IsNotExist(err) {
  			return "", nil
  		}
  		return "", err
  	}
  	return string(content), nil
  }
docs: // loadCheckpoint reads a string (which is assumed to be an authorization ID)
  // from the file at the provided path and returns it to the caller. If the
  // file does not exist an error is not returned and the returned ID is an
  // empty string.