What are 'unavailable tracks'?

[simon-weber]

This seems to be a global property of a track, which is carried over to playlists.

For example, this has caused issues in testing:

add to playlist:
{u'songIds': ['<1 songs>'], u'playlistId': u'f6f34bd2-44ae-425c-b1f4-4a72bff179f1'}

load playlist:
{u'continuation': False, u'playlist': ['<0 songs>'], u'playlistId': u'f6f34bd2-44ae-425c-b1f4-4a72bff179f1', u'unavailableTrackCount': 1}

Is this a property of tracks, or of playlist entries?