bugfix for worker error event, so it outputs error to callback

as reported in https://github.com/pgte/simple-redis-safe-work-queue/issues/11

this pull only does these 3 things:

1. fix the `worker error` event callback to return error
2. add a few tests to confirm `worker error` and `work done` are firing properly
3. edit doc to show that `work done` is returning `work` instead of `work.payload`