raise Http404, not return
Merge pull request #436 from bcipolli/raise_not_return

raise Http404, not return
Merge branch 'release-0.10' of github.com:learningequality/ka-lite into subtitles_immediate_fix
Merge branch 'subtitles_immediate_fix' of github.com:dylanjbarth/ka-lite into subtitles_immediate_fix
only update currently available videos for download of subtitles.
removing unintended comment
new_only now renamed as 'existing videos'.  No update is actually done.
Merge branch 'subtitles_immediate_fix' of github.com:dylanjbarth/ka-lite into subtitles_immediate_fix
updating srts list
fixing bugs, small and large, trying to improve outputted info (so we know what was actually downloaded.
fixing broken request url
Merge branch 'subtitles_immediate_fix' of github.com:bcipolli/ka-lite into subtitles_immediate_fix
Merged bcipolli's front end updates with my script fixes