Alert on failing Nginx requests

Based on #608, we want to be notified if (especially not user-controlled) requests fail.  

Currently, our main source of truth for alerts is Sentry. Next to that, we also use Icinga. Grafana Alerts would be the simplest solution in this case. However, is it worth introducing a third source of truth? Also, in the past, we had no option to deploy Grafana Alerts with Sentry.