More error message fine-tuning.

- Make sure errors are not overwritten in the executor.
- Do not report a process result for canceled processes. Such bogus
  errors currently spam the output, making it difficult to find
  the actual error that caused us to cancel the process in the first
  place.

Change-Id: I76d724d084532fbce243bf180d955d6f34ac0a91
Reviewed-by: Joerg Bornemann <joerg.bornemann@digia.com>