fix mpv thread not terminating when quitting from start screen + error handling to play_at_index()

[4shadoww]

For some reason mpv thread won't quit after quitting from start screen. After researching I didn't find better fix for this than simply creating mpv object after start screen.

[4shadoww]

Pushed another commit which adds error handling to YoutubePlayer.play_at_index()

[vaibkumr]

I just happen to add an exception at play_at_index(). For now, I skipped the current song and play next by printing an ugly message on the screen which breaks the UI (will replace it with urwid floating box soon)
I am not sure why main thread does not exit but your fix seems amazing. Can you please make the pull request for it again?
EDIT - I fixed the conflict, we were doing the same thing. It now quits on the start screen, thanks to you!