Improve feedback from Prosody on subscriptions

@MattJ said on chat:

Except for a small fix I need to deploy, it answers 200 or 500 with some text explaining the problem Currently it is returning 200 + error text in some cases, instead of 500

For djangoldp-packages/djangoldp-notification!33 (merged) we would need detailed message for possible errors.

A 200+error can be accepted in case where Prosody faced 404.

@MattJ Could you estimate how much time you would need, if you face any questionable one, feel free to ask which behavior should be applied.