Alerting: Notification channel http api fixes (#16379)

Fixes so it's possible to create new notification channel and providing uid.
Fixes better error/result handling when updating a notifcation channel.

Fixes #16372
Ref #16219 #16012

pkg/services/sqlstore/alert_notification.go

@@ -382,6 +382,8 @@ func UpdateAlertNotificationWithUid(cmd *m.UpdateAlertNotificationWithUidCommand
 		return err
 	}
 
+	cmd.Result = updateNotification.Result
+
 	return nil
 }