Gracefully pass over video downloads that errored out with 404's.

This is high priority, as the current download experience (due to many video download links not existing on KA's S3 servers) is quite poor. The frontend currently does not automatically skip over videos that were missing, and hence the process appears to the user to be stalled, until the page is refreshed.

Now, the `check_video_download` endpoint returns `None`/`null` instead of 0, for the video's progress, if it has been skipped, so that the frontend knows to skip it and start asking about the next video in the sequence.