swh.scheduler.backend_es: Return operation failure instead of raising

Prior to this commit, an error would raise and stop all indexation.
As the code is already waiting for a tuple (operation-status, item),
we instead leverage this and continue working on indexation.

All items whose operation-status is False (meaning failure to index,
whatever the reason) is not indexed. Another run would then pickup
the leftover and index it.

Related T986