[1.1.X] Fixed #12302: Modified force_unicode to avoid raising unicode errors when

handed exceptions with non-ASCII bytestring data and no working unicode method
under Python 2.6 and higher.

r12621 and r12622 from trunk.


git-svn-id: http://code.djangoproject.com/svn/django/branches/releases/1.1.X@12623 bcc190cf-cafb-0310-a4f2-bffc1f526a37