ClusterHealthResponse.validationFailures now maps to RoutingTable.allFailures()

The ClusterHealthResponse.validationFailures is currently wired to RoutingTable.failures , which contains cluster level validation errors. That means that the list doesn't contain any index level validation failure, if exists. Since we by default return only the top level information (`level=cluster`), this important information is hidden to the rest layer. This PR adds the index validation failures to this list.  The Java API always return index level information but one needs to check it for every index, which means this change is imho good here as well.