player: exit(1) if usage is displayed ( improperly invoked app ), exit(2) if can not load song
CORE/Playlist: because setNextSongByNumber is called from MIDI thread we can not load song here, instead call it from main ( e.g. GUI) thread - introduce Playlist::loadSong method for this. This mean that new song from playlist will be called after PLAYLIST_LOADSONG will be recaived. Also remove old/current song directly in Hydrogen::setSong method. CLI: update for this approach.