Fix disk checks in “gnt-cluster verify”

Tests have shown that the changes in commit b8d26c6e don't work as
wanted. If any disk wasn't found on the node, all disks located on the
same node would show as faulty. The cause was incorrect exception
handling on the node.

This patch changes the RPC call to return a per-disk success/error
status, avoiding the problem.

Signed-off-by: Michael Hanselmann <hansmi@google.com>
Reviewed-by: Luca Bigliardi <shammash@google.com>