Fixed #17810. Catch session key errors.

Catches memcached session key errors related to overly long session keys.
This is a long-standing bug, but severity was exacerbated by the addition
of cookie-backed session storage, which generates long session values. If
an installation switched from cookie-backed session store to memcached,
users would not be able to log in because of the server error from overly
long memcached keys.



git-svn-id: http://code.djangoproject.com/svn/django/trunk@17795 bcc190cf-cafb-0310-a4f2-bffc1f526a37