Fixed #5350 -- Added fallback to default 404/500 handlers when they're not...

Fixed #5350 -- Added fallback to default 404/500 handlers when they're not explicitly specified (or imported) in a urls.py file. Thanks to Thomas Güttler for the report and initial patch.

git-svn-id: http://code.djangoproject.com/svn/django/trunk@13590 bcc190cf-cafb-0310-a4f2-bffc1f526a37