catching TypeError hides root cause

In _operation.py, catching the TypeError can hide errors in toDataForJSON() methods defined for new custom types. It would be nice to either log the the original traceback or include some of its info with the new TypeError.

        try:
            import pdb; pdb.set_trace()
            json_representation = simplejson.dumps(
                result, cls=ResourceJSONEncoder)
        except TypeError:
            raise TypeError("Could not serialize object %s to JSON." %
                            result)