Fix backend.VerifyNode behaviour for VG problems

In case LVM is broken, backend.GetVolumeList will raise an RPC exception
(as expected since it's a function exposed over RPC). Therefore we must
be prepared to catch any such exceptions, so that we don't fail the
whole verify call in this case. cmdlib is already prepared to handle
string results for this response key.

Signed-off-by: Iustin Pop <iustin@google.com>
Reviewed-by: Guido Trotter <ultrotter@google.com>