Remove infinite loops from management commands

In the past, we've blocked IP addresses of users whose management commands were continually pinging our servers.  Turns out, that behavior is by design:   
* Any error in videodownload or subtitledownload causes the command to restart itself.
* When the same commands are restarted, it starts on the same videos.

Changes:
* Do not call force_job (infinite loop call) on error.  Instead, leave it to the user to press the "retry" button in the video_download interface (or to reload the page)--both of which start the job again, once it's completed.
* When an error occurs, do not simply abort--instead, continue to other videos / subtitles in the list.  Do not exit until all videos / subtitles have been tried once.