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>
sheep/request: straightforward error message in sheep_exec_req

Current message as below in sheep_exec_req is rather confused.

ERROR [rw 6471] sheep_exec_req(1133) failed No object found

In fact, I don't know which node sends the request and what it
wants to do. They are the minimum requirements for any investigation.

I think this is very helpful to diagnose an issue.

ERROR [rw 16160] sheep_exec_req(1135) failed No object found,
remote address: 127.0.0.1:7002, op name: GET_HASH

Signed-off-by: Ruoyu <liangry@ucweb.com>
Signed-off-by: Liu Yuan <namei.unix@gmail.com>
lib/event: fix and unify error message

Signed-off-by: Ruoyu <liangry@ucweb.com>
Signed-off-by: Hitoshi Mitake <mitake.hitoshi@lab.ntt.co.jp>