LUClusterVerify: Complain if disk is marked faulty

This will show a warning if, for example, one side of a DRBD
disk becomes unavailable. The data is collected separately
from the other verification data.

Example output:

* Verifying instance status
 - ERROR: instance inst1: disk/0 on node2 is faulty

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