Fixed #16516 -- Relaxed the blocktrans rendering a little by falling back to...

Fixed #16516 -- Relaxed the blocktrans rendering a little by falling back to the default language if resolving one of the arguments fails, raising a KeyError. Thanks, Claude Paroz and Aymeric Augustin.

git-svn-id: http://code.djangoproject.com/svn/django/trunk@16723 bcc190cf-cafb-0310-a4f2-bffc1f526a37