In SessionFileStore.__getitem__(), the store raises a
KeyError on corrupted session data, and leaves the
corrupted data on the filesystem.
If corrupted data is written during a bad appserver
shutdown, the SessionStore continually throws those
KeyErrors for any attempt to iterate over active sessions.
The SessionFileStore should at least raise a different
type of error indicating the problem is not KeyError but
corrupted data. But then shouldn't it clean up the
session? or maybe move it to a "corrupted" directory for