Fixed #17810 (again). Catch session key errors.

The previous commit didn't work with PyLibMC.
This solution appears to be the best compromise
at this point in the 1.4 release cycle.



git-svn-id: http://code.djangoproject.com/svn/django/trunk@17797 bcc190cf-cafb-0310-a4f2-bffc1f526a37