add resilience in case of 404 on variant playlist, discard playlist and try another one

If there is a variant main playlist and loading one of the variant playlists fails (e.g. 404 response) then the player stops instead of trying one of the other variants.

I quickly tried changing to another variant and removing the failed variant from _levels. It seemed to work with the chunks from the other variant being loaded but playback never started and the video was frozen. Before looking into it in more detail I thought it better to discuss it first to see what's the best way to solve this.

Start at variant playlist loading and the _levels handling described in the issue, tracing what happens after a 404 response. Confirm that a failed variant is discarded, another variant can load its chunks, and playback starts rather than leaving the video frozen.

Written by the indexing model from the issue text.