Made failed NC call error messages more informative

Fixes EUCA-9130
Fixes NC to report disk usage more accurately (via DescribeResource())

Specifically, disk usage for EBS instances that do not have any
ephemeral disk is now reported accurately.

Fixes EUCA-9452
Merge branch 'testing' of git+ssh://git.eucalyptus-systems.com/eucalyptus into testing