If a media item is disabled and page becomes a 404 response, player should handle it for already loaded pages

[tjenkinson]

Currently if the media item is disabled/deleted the player info endpoint will return a 404 instead of json, and this is ignored meaning the player keeps its previous state.

If the player was showing a live stream this means viewers will still see the livestream until they refresh the page.

In the case where a media item becomes unavailable the player should revert back to an ad saying "Unavailable" or something.