Add HTTPCode to APIError

[tritone]

APIError was intended to be transport-agnostic, so it currently doesn't have any fields for HTTP or gRPC response codes. However, will lead to some clunky error handling code for users once we use APIError on apiary library calls. To extract the most important fields, users will have to call errors.As twice with two separate error types:

if err := bucket.Create(ctx, "bad-project-id", nil); err != nil {
        var aErr *googleapi.Error
        if errors.As(err, &aErr) {
                fmt.Printf("Status: %d\n", aErr.Code)  // pulls HTTP response code
        }
        var apiErr *apierror.APIError
        if errors.As(err, &apiErr) {
                fmt.Printf("Reason: %s\n", apiErr.Reason())  // pulls error reason
                fmt.Printf("Details: %sn", apiErr.Details())
        }
        // Handle error
}

Code is in fact the only field from googleapi.Error that we are currently missing out on in APIError. For manual clients such as storage where most users will be HTTP-only, it would be cleaner and easier to document if we could add a method to retrieve this into APIError as well.

It may also be nice to pull the gRPC status into APIError as well; however, I think it's preferable to keep these as separate methods (HTTPCode and GRPCCode say) in order to avoid having to map between HTTP and gRPC statuses.

[cecobask]

+1 on this feature request

[ribrdb]

It seems like the json errors should include a grpc status code too, but the code is currently ignoring it:
https://pkg.go.dev/github.com/googleapis/gax-go/v2/apierror/internal/proto#Error_Status

It seems like the apierror should return that, or unknown if it's not included. Instead APIError.GRPCStatus() is returning nil for http errors, which means "OK".