Fixed #5640 -- Added some extra error reporting when smart_unicode() or

force_unicode() raise a UnicodeDecodeError. This should at least help people
identify which is the bad piece of data they passed in. About the best we can
do here.


git-svn-id: http://code.djangoproject.com/svn/django/trunk@6649 bcc190cf-cafb-0310-a4f2-bffc1f526a37