[1.2.X] Fixed #15535 -- Stopped the blocktrans template tag from raising a...

[1.2.X] Fixed #15535 -- Stopped the blocktrans template tag from raising a KeyError if an included variable can't be found in the context. Thanks, melinath.

Backport from trunk (r15709).

git-svn-id: http://code.djangoproject.com/svn/django/branches/releases/1.2.X@15710 bcc190cf-cafb-0310-a4f2-bffc1f526a37