Fix error when changing course details on split

LMS-11287. @dmitchell @nasthagiri please review.
I think old mongo doesn't respect the `allow_not_found` arg, acting as if it is always `True`.

    result = self.collection.update(
        {'_id': location.to_deprecated_son()},
        {'$set': update},
        multi=False,
        upsert=True,
        safe=self.collection.safe
    )
    if result['n'] == 0:
        raise ItemNotFoundError(location)

The upsert guarantees that n is at least 1, so the `ItemNotFoundError` is never raised. This is a bug.