[IMPROVED] Expose JetStream API error codes to user for better error handling

[edandavi]

Following this discussion with @wallyqs
this change allows the user to handle all the various jetstream API errors the server may return
like so:

if errors.Is(err, jetstream.JSErrCode..) {
  // do something
}

[piotrpio]

Sorry to be late here - I think this is a good idea, although I'm wondering if it wouldn't be convenient to have those error codes generated from server (as the server has all error codes listed in json anyway): https://github.com/nats-io/nats-server/blob/main/server/errors.json

We could have a template just for generating error codes and we would not have to worry about manually keeping the list up to date.

@edandavi would you be up to doing that?

[edandavi]

Hi @piotrpio,

Sure, I can do it,
but how would we sync the JSON file from the server repo to the client repo?

A possible solution could be to separate the errors JSON into a common mini-repository that would hold the errors, but that would require changes in the server repo too.
Or to manually copy it and then generate, which kind of beats the purpose here, I guess…