Playlist items are hidden if track lookup fails

[jcass77]

When browsing playlists containing URI's to tracks that are no longer available in the library, MMW will fail silently and not display any of the available information for that track.

The failure occurs in

mopidy-musicbox-webclient/mopidy_musicbox_webclient/static/js/process_ws.js

Lines 189 to 193 in 01ae68e

	var newplaylisturi = resultDict.uri
	playlists[newplaylisturi] = {'uri': newplaylisturi, 'tracks': []}
	for (i = 0; i < trackUris.length; i++) {
	playlists[newplaylisturi].tracks.push(tracks[trackUris[i]][0])
	}

.

The desired behavior should probably be to log the error to the console and fall back to using the track Ref instead. Since the track also will not be playable, it might make sense to disable the context menu for the affected tracks as well, and maybe add '[Not playable]' to the track name?