Fix handling of cluster verify hooks

The change to enforce boolean results for cluster verify group opcode
missed the HooksCallBack, which uses a very ugly 1/0
logic. Furthermore, the logic is wrong, since it unconditionally
resets the verify result to true.

The patch is changed to simply treat hook failures as failures, and do
nothing for offline/nodes.

Signed-off-by: Iustin Pop <iustin@google.com>
Reviewed-by: Michael Hanselmann <hansmi@google.com>