Symbolic constants for nats-server API error codes

[davidmcote]

Background

The nats-server package maintains a list of possible error codes at https://github.com/nats-io/nats-server/blob/main/server/errors.json.

The elements of this JSON listing appear to be ErrorsData structures which feed a code generator to produce go-native symbolic constants at https://github.com/nats-io/nats-server/blob/main/server/jetstream_errors_generated.go.

In NATS client packages however, there doesn't seem to be a complete list of error codes that may be returned from nats-server.

Without these constants, clients interested in detecting specific server-side error conditions must hard-code a subset of errors.json into their application.

Proposed change

Generate symbolic constants for NATS client libraries from the NATS server errors listing at error codes at https://github.com/nats-io/nats-server/blob/main/server/errors.json

Use case

Clients interested in detecting specific server-side error conditions can match error codes by named symbolic constants without first duplicating the information in https://github.com/nats-io/nats-server/blob/main/server/errors.json.