Client side basic API-version error handling

So far we have deferred looking at API versioning comprehensively (#317), things are still moving fast, and it hasn't been worth the complexity.

Debusine is currently going to ship in Trixie. Should we at least try to get something basic into the trixie client to allow it to gracefully report that it's out of date.

I'm thinking that we define an ancient-client response error code and add basic support for handling these to the client. In the future, (when APIs have versions), the server can emit these errors when it sees an un-versioned request.

The client can then display a useful error rather than a stack trace.

This won't help with API URLs that move, but all I can think that makes sense for 404s is to report "maybe your client is out of date or you have misconfigured it?"