How to return error ?

I would like to return something like this:
```
  3. loader                                            

     read 8, error 5, rejected 1
time: 5.97s
```

Currently I only found that we could return `RejectionResultBucket` or `AcceptanceResultBucket`. These two ways to return results only increment the "read" and the "rejected". How can I do to return errors ?