Improve cluster verify with hypervisor errors

In case the hypervisor has issues on one node, currently
backend.VerifyNode will exit via an exception (two exit paths possible,
one via HypervisorError from hypervisor.Verify(), and one via RPCFail
from GetInstanceList). This is bad as it invalidates all other checks of
that node.

This patch catches these two errors and allows the rest of the
VerifyNode function to run. This leads to a more complete verify cluster
run, for example now only real missing LVs are reported, not all of
them.

The cluster verify is not perfect as it will skip some tests even if it
has data, but this will require a more complete rewrite (see issue 90).

Also, the patch fixes and improves some error messages in cmdlib.

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