[1.2.X] Fixed #14870 -- don't catch AttributeErrors in database router...

[1.2.X] Fixed #14870 -- don't catch AttributeErrors in database router methods, instead check that the method itself exists.  Thanks to jonash for the patch. Backport of [14857].

git-svn-id: http://code.djangoproject.com/svn/django/branches/releases/1.2.X@14858 bcc190cf-cafb-0310-a4f2-bffc1f526a37