Fixed #14381 -- Clarified exception handling when instantiating Routers....

Fixed #14381 -- Clarified exception handling when instantiating Routers. Thanks to dauerbaustelle for the suggestion and patch.

git-svn-id: http://code.djangoproject.com/svn/django/trunk@14005 bcc190cf-cafb-0310-a4f2-bffc1f526a37