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.
in sf 2.6 they fixed things so that json requests get back a json error rather than html. did you check if this works here, and if things work with both 2.3 and 2.6?