sheep: exit when vdi bitmap collection is failed

In our test environment, we found an inconsistency of vdi bitmap
caused by failed colleciton process. The inconsistency is really
critical so calling panic() is much safer.

Reviewed-by: Saeki Masaki <saeki.masaki@po.ntts.co.jp>
Signed-off-by: Hitoshi Mitake <mitake.hitoshi@lab.ntt.co.jp>