rgw/multisite: fix ret handling in bucket read_sync_status()

[smanjara]

Fixes: https://tracker.ceph.com/issues/59576

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[smanjara]

without the fix:

radosgw-admin bucket sync markers --source-zone zg1-1 --bucket test-bucket

ERROR: sync.read_sync_status() returned error=0

with the fix:

[ { "key": 0, "val": { "status": "incremental-sync", "inc_marker": { "position": "", "timestamp": "2023-04-03T17:40:46.678211Z" } } },

[smanjara]

http://qa-proxy.ceph.com/teuthology/smanjara-2023-05-17_20:38:44-rgw:multisite-wip-smanjara-testing-distro-default-smithi/7276931/teuthology.log

unrelated failures.

[smanjara]

jenkins test make check