UI when a session that is not ready (or does not have web ui) is accessed

[olevski]

The idea here is that we could have a proxy or something similar that will always handle the HTTP access to the session.

So as long as this sidecar container (or component) is running then when the user goes to the session url they will get a nice output. I.e. not just a 503 code or something weird. When the session does not have a http frontend we can mention that. And if the session is failing we can give information about the failure.

This should be considered/implemented only after #614 and #637.