Ah seems my clipboard borked up on saving the post
Embed is here
Then you would have to (do as you describe) poke the videos API for the video by ID and see if it’s dead for your self, then decide what to do from there.
Twitch/the embed won’t do it for you
Your post sounds like a feature request not a third party implementation query. Feature suggestions should go to https://twitch.uservoice.com/ what you describe (doing something else on video by VideoID not existing), would be quite a change both for developer (confusion) and Twitch itself.
(Discord/Forums would get a lot of “I put this video ID in and something else appeared instead” as people wouldn’t realise that it does something different (show a different video) on a dead video)