bug fixes and stability improvments

• fix sudden crash on no watermark downloading
• continue downloading from where it stopped
• support no watermark downloading for users with more than 1000 video

the downloading still slow, we will work on downloading performance

clear error messages

I just handled the common errors with clear messages

change no watermark structure

v2.0.0

Update README.md

first release

This is the first release of this script. Feel free to write any suggestions you think about.