rgw: don't turn 404 into 400 for the replicalog api

404 is not actually a problem to clients like radosgw-agent, but 400
implies something about the request was incorrect.

Backport: dumpling
Signed-off-by: Josh Durgin <josh.durgin@inktank.com>
Merge pull request #804 from jdurgin/wip-rgw-replica-log-next

rgw: don't turn 404 into 400 for the replicalog api

Reviewed-by: Yehuda Sadeh <yehuda@inktank.com>