async_task: avoid false negative JobNotFoundException

[bossie]

Commit dfe465a for #240 added these lines to avoid failing and reporting an (expected) error when a user decided to delete his batch job while async_task is actively polling SHub:

openeo-geopyspark-driver/openeogeotrellis/async_task.py

Lines 247 to 250 in 0444fb4

	except JobNotFoundException as e:
	# TODO: look for "Deleted ..." log entry in Elasticsearch to avoid a false negative?
	_log.warning("job not found; assuming user deleted it in the meanwhile", exc_info=True,
	extra={'job_id': e.job_id})

This approach is maybe too drastic and could cover up an unexpected JobNotFoundException.

The batch job itself is obviously gone and the only way to tell if the user actually deleted it himself is by inspecting the logs in ES.

Cost-benefit etc.