Add json parsing error stat for namerd

[stevej]

When we fail to parse JSON from the k8s namer (or I suppose any namer, really), we should make sure to increment an error counter.

This would have made resolving #1361 a little easier.

(aside: this would be a good starter bug for one of our new peeps)

[adleong]

"fun" fact: for some reason there's no parsing error when we attempt to parse a string into an Int typed field, the object with the Int field is happily constructed. It's only much later when we attempt to use that field that a ClassCastException suddenly appears because (surprise!) it was a String all along.