code: func isBadKeyError(err error) bool {
  	if err == nil {
  		return false
  	}
  	// See https://go.googlesource.com/oauth2.git/+/197281d4/internal/oauth2.go#32
  	// Unfortunately, if uses fmt.Errorf.
  	s := err.Error()
  	return strings.Contains(s, "private key should be a PEM") ||
  		s == "private key is invalid"
  }
docs: // isBadKeyError sniffs out errors related to malformed private keys.