Since we have no corrupted sessions on the disk, the Session KeyError is still appears.
KeyError appears if SessionFileStore can't unpickle session while loading it from disk.
Session can't be unpickled only if it has objects with buggy implemented unpickling feature.
Developer, who works under webkit, would like to fix unpickling for his session.
So, match better for developer to see the original exception instead of KeyError.
I hope, patch #678129 to be in time for final release 08.