Unify "unknown session ID" and "session is closed" page?

[martinpitt]

@marusak had the idea to make these two cases spit out the same page. This would allow us to clean up the SESSIONS map aggressively, right after a session is gone. See issue #30 and #54.

It still feels like these are two different cases -- trying a random (nonexisting) UUID shouldn't claim that "the session has ended" -- that would be equally confusing.

I didn't quite make up my mind about this, but wanted to note it down.