A RunControl is re-runnable if all its workers are,
a RunWorker is re-runnable if it's Stopped and unless it
says otherwise.

Also ensure SimpleTargetRunner only reportStop() once
per run and make process error message re-usable.

Change-Id: I73f5fb724d3026ceb81d5e32a3a71b4814b2bca9
Reviewed-by: Christian Stenger <christian.stenger@qt.io>