Provide more input data if unmarshaling fails

[lag13]

I think it would be nice for debugging purposes if when unmarshlling fails in the JSON function more of the input data being unmarshalled was returned in the error message. Maybe its not necessary but it feels nicer to look at an error message like: unmarshalling response body, the body is '<this>is a really weird body': '<' looking for beginning of value instead of: unmarshalling response body: '<' looking for beginning of value.