Add a NotOK counter

A client-only release.

Adds a NotOK counter for responses with a status-code != 200;

$ kahttp -address http://localhost:9990/no -timeout 3s | jq .
{
  "Started": "2020-12-26T15:15:33.964883773+01:00",
  "Duration": 2978245599,
  "Rate": 10,
  "Clients": 1,
  "Dials": 30,
  "FailedConnections": 0,
  "Sent": 30,
  "Received": 30,
  "Dropped": 0,
  "FailedConnects": 0,
  "NotOK": 30
}