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.
i think this provider actually looks at the siblings, not at the ancestor. it goes to the parent of the requested url and tries to find a route there. if it finds, it generates the children of that parent.

so indeed, we can't make this and the parent provider the same class. but this class is misnamed, it should be SiblingsSuggestionProvider i think.