Add custom error page
The old webredirector has an advanced, but outdated custom error page: https://gitlab.cern.ch/webservices/WebRedirector/-/blob/master/404.aspx
Our new webredirector currently just returns a basic error page: https://gitlab.cern.ch/webservices/web-redirector-v2/-/blob/dev/app/api/handlers.go#L26
We should have a similar "nice" (user-friendly) error page. For a start we can copy the old one, then later we can update it (maybe with a similar theme as the webservices portal?).