Return more details when request is malformed

[piranha]

Currently the only error client gets is "Malformed JSON in request body.", while Jackson's exceptions contain enough details to diagnose a problem (i.e. "Unrecognized token 'ture'").

I could not come up with a PR solving this w/o majorly reordering internals, since json-body-request indicates an error happening with an empty response. If you're interested in that, I can work on a PR given the idea of a change. :)

[weavejester]

This seems like a reasonable improvement.