Add Playlist method #last to get last ID in the system.

This project really needs an ORM...
Playlist fetch returns empty JSON response when 404.
Exclude exception-related tracks from client parse.

Maybe there is a problem on the server, but the client should keep on chugging.
Request testing uses configured port.
Fix a request test and change label for another.
POST request tests use configured port.
YouTube track fetching can detect unstreamable tracks.
Fix failing router tests.
Error stack is useful.