code: func WithMessage(e error, msg string) Error {
  	if e == nil {
  		return nil
  	}
  	return WrapSkipping(e, 1).WithValue(message, msg)
  }
docs: // WithMessage returns an error with a new message.
  // The resulting error's Error() method will return
  // the new message.
  // If e is nil, returns nil.