code: func Message(e error) string {
  	if e == nil {
  		return ""
  	}
  	m, _ := Value(e, message).(string)
  	if m == "" {
  		m = Unwrap(e).Error()
  	}
  	return m
  }
docs: // Message returns just the error message.  It is equivalent to
  // Error() when Verbose is false.
  // The behavior of Error() is (pseudo-code):
  //
  //     if verbose
  //       Details(e)
  //     else
  //       Message(e) || UserMessage(e)
  //
  // If e is nil, returns "".