Failed jobs should be able to return an object in the job callback

When failing a job by passing a parameter into the first argument of the `done()` callback, this object usually gets converted to a string. It would offer more flexibility to the pass back the object without calling `toString()`.

Here is the code that converts the error parameter into a string
https://github.com/Automattic/kue/blob/master/lib/queue/worker.js#L126

Passing an object on success does not get converted, see https://github.com/Automattic/kue/blob/master/lib/queue/worker.js#L193

As a workaround, one can fail a jobs like this in order to get an object passed back

```
done({message: {foo:'bar'}});
```

Start in lib/queue/worker.js at line 126, where the failed-job callback parameter is converted to a string, and compare it with the success path around line 193. Verify the behavior using the existing worker or job callback tests, and consider the issue done when an object passed to done() on failure is returned without being stringified.

Written by the indexing model from the issue text.