add reason code to slack notifier

this uses the new error with reason to determine based on status code what the reason is for the slack integration

partial #3231

notify/slack/slack.go

@@ -213,5 +213,7 @@ func (n *Notifier) Notify(ctx context.Context, as ...*types.Alert) (bool, error)
 	// https://api.slack.com/changelog/2016-05-17-changes-to-errors-for-incoming-webhooks
 	retry, err := n.retrier.Check(resp.StatusCode, resp.Body)
 	err = errors.Wrap(err, fmt.Sprintf("channel %q", req.Channel))
-	return retry, err
+	reasonErr := notify.NewErrorWithReason(notify.GetFailureReasonFromStatusCode(resp.StatusCode), err)
+
+	return retry, reasonErr
 }