[Bug] 502 errors on map overview after ungeoreference sometimes

[mradamcox]

Describe the problem

@oxguy3 reported this series of actions:

1. start georeferencing but preview isn't loading (presumably due to [Bug] Some preview tiles are not loading in georeferencer view #160) then return to the map overview
2. click "ungeoreference" on this layer to completely start over
3. now the entire map overview page gets a 502 error.
4. after a minute or so, the 502s go away and everything is back to normal.

Expected behavior

No 502 errors.

Additional Context

I'm guessing that this is caused by an "orphan" session not being properly deleted during the "ungeoreference" process, specifically the session that had been created in step 1 above but then was abandoned. At this point, the session would stick around until the cleanup process deletes it, but accessing the session.lyr2 attribute, for example, would fail because the layer no longer exists.

A better way to handle this would be to introduce a "void" status for sessions, such that they don't need to be deleted when the "ungeoreference" or "unprepare" actions are taken. This would be ideal, but will have other implications that need to be thought out.