I just got a "remote-server-not-found" because I have s2s_secure_auth to true and the remote server didn't support encryption. It would be nice to get a description with this kind of errors.
remote-server-not-found pretty much only means that the host does not exist in DNS
remote-server-timeout covers all other issues
The tricky part here is that the code that sends the error bounces to clients is disconnected from the part that deals with s2s connection failure, and the actual cause is lost between them.
One small step
Further improvements for cases of certificate errors in https://hg.prosody.im/trunk/rev/698ff3610e57
Maybe this can be considered mostly completed now?