Possibly more useful way of logging missing thread error.
If the context is broken due to an unknown thread, treat the step as failed.
Prevents a constant stream of errors coming from:
https://trello.com/c/YU8dMM1K/158-mystery-serialization-error
Make PlaceholderTask.ownerTask be the WorkflowJob.
Fallback value should be this, not null.
ExecutorPickle should not use getOwnerTask unless it has to.
Merge pull request #21 from jenkinsci/PlaceholderTask.ownerTask

Set an owner task for placeholder tasks