Add verification of RPC results in _WipeDisks

Due to an oversight, the pause/resume sync RPC calls in _WipeDisks
lack the verification of the overall RPC status, and directly iterate
over the payload. The code actually doing the wipe does verify
correctly the results. This can result in jobs failing with a hard to
diagnose:

OpExecError ['NoneType' object is not iterable]

instead of proper "RPC failed" message.

This patch adds a hard check on the pause call, but for the resume
call it just logs a warning if the RPC failed; the rationale being
that if we can't contact the node for pausing the sync, it's likely
wiping will fail too, but after the wipe has been done, we can
continue.

Signed-off-by: Iustin Pop <iustin@google.com>
Reviewed-by: René Nussbaumer <rn@google.com>