In version 3.6, responses to isMaster would be first parsed and checked for errors, then sent to the hook for validation. This means that errors like HostUnreachable would be caught before entering hook->validateHost.

In 4.0, it seems we've inverted this ordering, which is leading to an error "Surprised to discover that X does not believe it is a config server" in step-down tests.

We should fix the ordering to be like it was in 3.6.