sheep: detect invalid response code in sd_strerror()

Commit a4d01de did wrong treatment of data structure of request. And
it caused segfault in sd_strerror() because of wrong index of the desc
array. This patch let the function catch such a bug and return string
representation of unknown response code.

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