DNS error rcode type

[sunfishcode]

The DNS-error-payload type is currently defined as this:

  /// Defines the case payload type for `DNS-error` above:
  @since(version = 0.2.0)
  record DNS-error-payload {
    rcode: option<string>,
    info-code: option<u16>
  }

This has the rcode value be a string. However, DNS rcodes have standardized values that are defined in terms of u16 values.

With the current type being string, implementations appear to be using ad-hoc error messages which don't correspond to any standardized rcode value.

[pchickey]

Also worth considering in wasi-sockets, which makes an ad-hoc mapping: https://github.com/WebAssembly/wasi-sockets/blob/main/wit/ip-name-lookup.wit#L42-L47

[sunfishcode]

Thinking about this more, I think having the precise u16 value isn't crucial, and at the same time, isn't easy for common implementations to provide. However in that case, the u16 info-code field is also not crucial, and also not easy for those same implementations. So I've now filed WebAssembly/wasi-http#204 to propose just having a simple string.

[lann]

This kind of detail seems like it might fit better in a more focused wasi:dns interface, which seems like a perfectly sensible thing to have anyway for anything that goes beyond getaddrinfo.