Ensure missing assets.json is an understandable error

Right now, I have updated the `.NET` and `python` shims to ascend looking for an `assets.json`, and if they _find_ one, then add that to the request to start playback.

What this means is that if an assets.json is _not present_, the proxy will return a `recording not found` error, but _not_ an restore error. There was no way to know that it was necessary to clone!

We need to ensure that when a recording is not found, we at the _very least_ link out to an aka.ms that describes what it would mean if they were _expecting_ an assets.json to restore and one did not.