When conduit call made through arcanist via `arc conduit-call some.call-here` ends up with error, e.g. following JSON is received
```
{"error":"ERR-CONDUIT-CALL","errorMessage":"ERR-CONDUIT-CALL: Method 'diffusion.jenkinshook' doesn't define these parameters: 'buildNumbera'.","response":null}
```
so `error` isn't empty, we should return non-zero exit code from Arcanist.