[1.2.X] Fixed #14381 -- Clarified exception handling when instantiating...

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

Backport of r14005 from trunk.

git-svn-id: http://code.djangoproject.com/svn/django/branches/releases/1.2.X@14008 bcc190cf-cafb-0310-a4f2-bffc1f526a37