sheep/ops: fix incorrect error log on remove_epoch

If epoch file is not existed due to the node had left, cluster format
command will cause the error in sheep.log

ERROR [main] remove_epoch(246) failed to remove
/tmp/sd9/epoch/00000002: Operation not permitted

Signed-off-by: Ruoyu <liangry@ucweb.com>
Signed-off-by: Liu Yuan <namei.unix@gmail.com>