Improved webservice error handling
- Introduce a 404 page for the cara webservice. It currently doesn't do too much - it just prints a message in the standard page template
- Introduce general error handling page. For example if the model raises an exception for some reason etc.
- If the voila-server raises a 404, we now redirect to the cara webservice, and get a nice 404
I'll assign to Nico next week, but won't do it now to avoid notifications.