[LocalVisage] Rebuild task continues running in the background after canceling #614

I've only tried it with the Rebuild Face Recognition Model task, but if you start the task, then cancel it by clicking the red X in the Task Queue in Stash, the python script continues to run in the background. If you then start it again, you'll have two instances of the Python script running. Rinse and repeat for even more Python instances. (I had to manually kill the processes when I was troubleshooting the zip file bug I reported)