@Override
        public Node toNode() {
            throw new IllegalStateException("Candidate node on " + parent.get() + " is invalid: " + invalidReason);
        }
A candidate which failed to transition from virtual to concrete.
It will silently stay invalid no matter which method is called on it.
node-repository/src/main/java/com/yahoo/vespa/hosted/provision/provisioning/NodeCandidate.java