Be more informative about problem directories being gone

[ernestask]

Follow-up from #205.

In cases where we find that the problem directory is gone, @marusak suggested disabling interaction (maybe not the button to delete, though) with the problem and providing an explanation for why that is the case.

Other suggestions are also welcome.

[ernestask]

Another thing that could be done is installing a GFileMonitor for the problem directories and watching for deletions or moves. That would allow changing the view state before the user can interact with anything, and not just when they click away and back.