sheep: some debug output enhancement

- do_epoch_log_read() will flood the ERROR message because epoch file not exist
  but this is a valid case when node have some epoch missing. So change it to
  sd_debug.

- enhance md_move_object() for better debug

Reviewed-by: Hitoshi Mitake <mitake.hitoshi@lab.ntt.co.jp>
Signed-off-by: Liu Yuan <namei.unix@gmail.com>