Fix misleading error message when checking disks

_CheckDiskConsistency outputs "Can't get any data from node NODE" when no drbd
is found on the target node. This causes a misleading error message to be
output for example on failover (when the primary node is down, or the instance
is not running), stating that no data could be got from the secondary node,
which scares the user and misleads him. Changing this to "Disk degraded or not
found on node %s" still reports that something is missing, but on the other
hand doesn't make the user think the node is down, or has no data at all...

Reviewed-by: imsnah