create custom exception/error handling mechanism

As a follow up from #177 

Would like to spice up the frontend test only by adding one more level in the routes/contents created in the FixtureLoader to see both ancestor and parent suggestions.
this means we can only have one single "default" provider, right?

can we validate here that the group names are unique? otherwise the error is only seen when a 404 is triggered, which could bite somebody that forgets to test his 404.