Fixed #14870 -- don't catch AttributeErrors in database router methods,...

Fixed #14870 -- don't catch AttributeErrors in database router methods, instead check that the method itself exists.  Thanks to jonash for the patch.

git-svn-id: http://code.djangoproject.com/svn/django/trunk@14857 bcc190cf-cafb-0310-a4f2-bffc1f526a37