Stopping a Ponos task in RQ marks it as failed

https://redmine.teklia.com/issues/8010

When a Ponos task in RQ is marked as Stopping, the container is stopped then the task is set to Stopped. However, almost immediately afterwards, the container's exit code is checked. If it is non-zero, the state becomes Failed, even if the task was manually stopped. So stopping a task always appears to mark it as Failed instead.