deleted assets: thumbnail flashes before showing error

[dylanjha]

Repro steps

• upload a video
• view the video
• delete the asset from Mux
• try to view the video
• see that the thumbnail flashes before showing an error message

Internally, this is what's happening:

• video player loads with poster= attribute
• even though the asset is deleted, previously rendered thumbnail images are cached by cdns for a while (days), so the request to load the poster image (thumbnail) succeeds
• the m3u8 playlist 404s, which triggers the correct error "This video does not exist"

Solution -- maybe there's something better but this is the best idea I have:

• before rendering the video player do a fetch GET to the m3u8, if it 404s then fallback into the error state