Fall back to h264 if av1 isn't available

[ArtyIF]

describe the feature you'd like to see When a video (usually a short, low-quality one. a rather embarrassing example) doesn't have the AV1 version, Cobalt just gives up and shows an error. However, when you go into settings and change it manually to H264, the video downloads just fine. My suggestion is to warn the user if the AV1 version isn't found, and then suggest looking for other versions. This might also probably apply to VP9, but I haven't checked.

describe alternatives you've considered Changing it manually, but it's not the best method, obviously.