Fix node Get() error code

Issue #2998
Yes, it was.

```
curl localhost:8080/api/v1beta1/minions/127.0.0.10
{
  "kind": "Status",
  "creationTimestamp": null,
  "apiVersion": "v1beta1",
  "status": "Failure",
  "message": "minion \"\" not found",
  "reason": "NotFound",
  "details": {
    "kind": "minion"
  },
  "code": 404
}
```

Should be
```
curl localhost:8080/api/v1beta1/minions/127.0.0.10
{
  "kind": "Status",
  "creationTimestamp": null,
  "apiVersion": "v1beta1",
  "status": "Failure",
  "message": "minion \"127.0.0.10\" not found",
  "reason": "NotFound",
  "details": {
    "kind": "minion"
  },
  "code": 404
}
```