Connaisseur fails (500) if rejecting an image due to failure to send alerts

[Starkteetje]

Describe the bug See observation https://github.com/sse-secure-systems/connaisseur/pull/873#discussion_r1095759341

There's an option that allows to reject a deployment if sending the corresponding alerts did not work. If so, Connaisseur throws an error and rejects the deployment (as desired). However, the error displayed isn't meaningful and the HTTP response (500) isn't meaningful either.

Expected behavior Connaisseur fails with 4xy/5xy error code and a message that highlights the error was due to sending alerts, not due to failed trust data.