Make some fixes to logging in PVF subsystem (#6180)

* Log exit status code for workers

* Make log for execute job conclusion match prepare job conclusion

Trace log for conclusion of prepare job:

```rs
gum::debug!(
	target: LOG_TARGET,
	validation_code_hash = ?artifact_id.code_hash,
	?worker,
	?rip,
	"prepare worker concluded",
);
```

Co-authored-by: parity-processbot <>