call callback of the current job with an error when the worker exits

When a worker dies while it was working on a job the callback for that job should receive an error so any cleanup can be performed.

My specific use case is with https://github.com/dannycoates/picl-idp/blob/signCheck/routes/sign.js#L67-L87

If the worker dies, the callback should still be called so that the http request can complete.

I'm sorry, I didn't add a unit test, they appear to be broken with node v0.10.21.