Fixed #12693 -- Improved error handling when there is an error setting up the...

Fixed #12693 -- Improved error handling when there is an error setting up the database router chain. Thanks to dhageman for the report and fix.

git-svn-id: http://code.djangoproject.com/svn/django/trunk@12305 bcc190cf-cafb-0310-a4f2-bffc1f526a37