[1.2.X] Removed reference to potential exception during template...

[1.2.X] Removed reference to potential exception during template rendering-time from the ungettext() documentation. Exceptions at that point should be considered and reported as a bug.

Backport of [14758] from trunk

git-svn-id: http://code.djangoproject.com/svn/django/branches/releases/1.2.X@14759 bcc190cf-cafb-0310-a4f2-bffc1f526a37