Improve CommunicationFailureNotificator message contexts

- Explicitly say it's a communication failure in _determine_message
  (case when service is available but returned some error),
  because some errors might look like a problem in clive itself when
  that's a error message received in the reponse.
- When no error details available, don't stringify exception, instead say
  "No error details available".
- More context for message when no response is available (so service
  seems not to be available).