Comment 1 for bug 999502

The relevant line is here: https://github.com/openstack/horizon/blob/master/horizon/dashboards/nova/instances_and_volumes/volumes/tables.py#L96

If you're getting that error there's something strange about the data being returned by nova volume (e.g. it finds an attachment but that attachment doesn't have an "instance" key as it should), however we could code that more defensively and have it fail more elegantly.