Add custom error pages

We should put in some effort to provide nicer error pages which also include the recent announcments from RWTH GitLab and RWTHjupyter.

See:

• https://github.com/kubernetes/ingress-nginx/tree/master/images/custom-error-pages
• https://kubernetes.github.io/ingress-nginx/examples/customization/custom-errors/

The current error page just displays: "500 Service Unavailable"