Check for temp files and offer to clean before starting

[hqhoang]

I started the script on my blender project file, realized a mistake and stopped the script shortly. There're partial temp files left in the output directory as a result (1-2 second videos). On the next run after some modifications to the video length, the second phase of course picked up those partial temp files and assembled them into the final video. Luckily I reviewed and spotted those weird 1-2 second flashes in the middle of the video before publishing it.

Since we can't reliably identify the filename generated by Blender, maybe we should attempt to detect any existing temp files before starting (at least they are pulverize_frames_*) and warn user to move or delete them?

I'll see if I have some time to make that improvement before someone else does :-)