Change BlockDev.Shutdown() failure result

Currently, the Shutdown() methods of block devices return True/False.
This doesn't permit any error detail reporting.

This patch changes the return type to None for success, and raises
BlockDeviceError in case of failure. This permits the details to be
passed up the stack.

For LVM and file-backed devices, this is a simple change. For DRBD, we
first remove the shutdown of disks in case of network activation
failures (since with static minors the minor is used anyway, we don't
gain anything by clearing it), and the we simply change _ShutdownAll()
to raise an exception.

Reviewed-by: ultrotter