You're attempting to get an error from the DataResponse object. A DataResponse object contains a collection of requests/responses, calling getError there doesn't make sense as there could be several errors -- which error would the response return?
http://code.google.com/apis/opensocial/docs/0.7/reference/opensocial.DataRequest.html
What you want to do is look for the error on the individual ResponseItem object, so dataResponse.get(whatever).getErrorMessage())
http://code.google.com/apis/opensocial/docs/0.7/reference/opensocial.ResponseItem.html