add missing result codes to sd_strerror

This also uses array to look up the error message for a bit better
performance.

Signed-off-by: MORITA Kazutaka <morita.kazutaka@lab.ntt.co.jp>
use sd_strerror to print error reason

Signed-off-by: MORITA Kazutaka <morita.kazutaka@lab.ntt.co.jp>
sheep: fix error message of do_epoch_log_read

Signed-off-by: MORITA Kazutaka <morita.kazutaka@lab.ntt.co.jp>