Fixed #12718 -- Tighten up the error handling when loading database routers....

Fixed #12718 -- Tighten up the error handling when loading database routers. Thanks to Jeff Balogh for the report and patch.

git-svn-id: http://code.djangoproject.com/svn/django/trunk@12336 bcc190cf-cafb-0310-a4f2-bffc1f526a37