Added a more verbose error message if response is 404

I spent a little while trying to work out what on earth SimpleGeo::NotFound meant, and thought it was some sort of issue with the gem setup (new machine) or what.

It was probably just me being a muppet, but I'd think this error message should make things a whole lot more obvious.

    SimpleGeo::NotFound in PlacesController#near

    The URL provided is incorrect. (404): Not Found - {"code":404,"message":"Not Found: /1.0/places/,.json"}