worker: treat 404 as an empty marker
worker: treat 404 as an empty marker

If the log does not exist at all, since no operations have been completed
on that bucket, we get a 404.

Signed-off-by: Josh Durgin <51652c783f70f5b272ebd02f1fd20d3af7a2449f@inktank.com>

ceph/radosgw-agent,ceph/radosgw-agent