Better exception message on converting between Json and InnerVal.

Currently, converting our inner class InnerVal into JSON is done by JSONParser.

I think there is lack of information why Exception has been thrown when converting InnerVal into Json(see innerValToJsValue method).

It would be better provide more informations as following.

1. what was the innerVal value we are trying to convert.
2. what was actual dataType of innerVal we are trying to convert.
3. what dataType this innerVal need to be converted into in Json.