Support for handling errors

[francislavoie]

An issue I noticed while reviewing the code is that on error, this handler just returns nil

caddy-extauth/extauth.go

Lines 81 to 86 in 2cb3268

	if err != nil || resp.StatusCode != http.StatusOK {
	// something went wrong or the server responded with something != 200 -> reject request with 401
	log.Error().Str("url", r.URL.RequestURI()).Msg("failed to authenticate")
	w.WriteHeader(http.StatusUnauthorized)
	return nil
	}

This means that users don't get an opportunity to handle the error case with the handle_errors directive: https://caddyserver.com/docs/caddyfile/directives/handle_errors

The way this should probably work is that you return caddyhttp.Error(http.StatusUnauthorized, fmt.Errorf("not authenticated")) instead, similarly to the caddyauth module:

https://github.com/caddyserver/caddy/blob/653a0d3f6bd7b66197abd1e00e366164876a9f2b/modules/caddyhttp/caddyauth/caddyauth.go#L88

Using caddyhttp.Error gives Caddy an opportunity to handle the error and do something else with it:

https://github.com/caddyserver/caddy/blob/653a0d3f6bd7b66197abd1e00e366164876a9f2b/modules/caddyhttp/server.go#L238

[trusch]

Thanks for your input! I'll adjust and test that.