raise Http404, not return

This was introduced in a previous checkin, migrating from HttpResponseNotFound (response) to Http404 (exception), in order to return API responses properly:
https://github.com/learningequality/ka-lite/commit/97d4de14ac16a5a9b2ff2d82aaee7bc748416cc5

Exceptions are raised, not returned.  Fixed here.

2 word change, very simple!