Fix to prevent barrage of errors on cancel event

Fix for #1490.

Summary of changes:
*Call force_job with launch_cron=False
*On raising DownloadCancelled in download_video function, raise again after catch to bubble exception up to handle function on the Command.
*Handle DownloadCancelled separate to ordinary exceptions to handle function:
**Set flags on video object to prevent future downloads
**Don't set progress as an error, as failure of the download is desired user behaviour.

Tested, seems to work, but could use a little more pummeling if @dylanjbarth or @aronasorman have time.