common/origin_save: Prevent erroneous save code now status overriding
Loading status for a save code now request is first retrieved from the scheduler database.
When a visit date for a save code now request has been found, the visit status is then checked to possibly update the final status of the request to failed (when an origin is not found for instance).
However to avoid sending too many costly queries to the storage, the visit date and status are only checked once and stored to swh-web database.
So ensure to not override a final loading task status (succeeded or failed) to avoid invalid save code now request reporting.
Migrated from D5378 (view on Phabricator)