worker: errors coming from lightning events are objects, not strings

When Lightning rejects an event, it sends a payload like this:

{
  errors: {
    [job-id]: ["field cannot be blank"]
   }
}

I think this has changed (probably a long time ago) because the error payloads used to be simple strings not_found

We need to update error handlers in the worker to be able to handle these error payloads. I think it's as simple as stringifying them.

This is particularly important on credential errors.

An immediate fix is to just dump the payload, but a better one would be to unwrap the message into a nice string and log it.

Later, we might want to feed these errors back into the fail reason returned by run:complete