Fix cluster resolver error handling

[zasweq]

We need to fix cluster_resolver error handing. The desired behavior as per gRFC A37 is to create a priority and send down an empty address list if an EDS Discovery Mechanism or DNS Discovery Mechanism errors before a first good update, triggering Transient Failure for that priority and causing fallback if another lower level priority. In master, we simply don't create the priority child in the EDS case and send down a resolver error in the DNS case.