Add slightly more expressive error. 

It isn't documented thoroughly that Cornice only deals with JSON requests.

Actually, it could quite simply support standard www-form, with the flattened convention from Colander, couldn't it ?

Anyway, I was debugging and it took a while to figure out it was
trying to decode my www-form into JSON.  This makes it a little more explicit.
Merge pull request #188 from abourget/patch-1

Add slightly more expressive error.