Error when running "scripts/run_preprocess.py" with custom video

[DenisKochetov]

Tries to download data plus deletes folder with custom video
When I try to preprocess custom video, its folder gets deleted and the programme is waiting for me to input url.

To Reproduce
Steps to reproduce the behavior:

1. Create folder database/vid_data/$seqname
2. Run python scripts/run_preprocess.py $seqname cat quad "0"
3. Get your folder deleted

Expected behavior
Preprocessing without an attempt to download the video.

Screenshots

Additional context
I fixed it with extra check for directory existence.

[gengshan-y]

Merged, thanks!