Add name of playlist to song not found file

[c0ball]

I think it would be good to also save the name of the playlist in which songs could not be found e.g.

Playlist 1:

• Song A
• Song B

Playlist 2:

• Song C
• Song D

[c0ball]

Maybe you can look into #90
I have implemented the logic to create a new file for every sync in which songs could not be found. I also grouped the songs by their playlist.