The reasoning is the same as in one of the previous commits. I think that
returning multiple values from one method can be confusing and potentially
error-prone. Also, this method will return ComposeResult soon too.
Also, I chose to create a new composeStatus struct instead of reusing
worker.JobStatus. I think that's a good thing in terms of consistency and
reducing the dependencies between packages.
Not a functional change.
5806e3ea08