Fixes Hyper-V volume discovery exception message

On some editions of Windows / Hyper-V server the SAN policy is set
by default to "Online All", bringing online any disk, local or shared,
attached to the host.

Since only offline disks can be attached as passthrough disks to a Hyper-V
VM, this prevents Cinder volumes from being attached to instances,
resulting in an exception that can be particularly hard to troubleshoot
without a clear knowledge of the context.

This patch includes some details in the exception message for
troubleshooting purposes.

Change-Id: I6d0b49812f8ca965ca53be34ef009997c8e79728
Closes-Bug: #1361419

Conflicts:
	nova/virt/hyperv/volumeops.py