Currently if there is an error (not found) the error printed out
is to do with the inablity to convert an empty body into the expected json.

This patch will fill in the err correctly.

example of before (with NotFound error):
$ kubectl top node
failed to unmarshall heapster response: json: cannot unmarshal object into Go value of type []v1alpha1.NodeMetrics

Now:
$ kubectl top node
the server could not find the requested resource (get services http:heapster:)