R1:d0013d08983a

Distinguish between unreachable cluster database hosts and missing MySQL databases

Summary:
Fixes T11577. When we connect to a host and try to select a database which does not exist, we currently treat it as though the host wasn't reachable.

This isn't correct, and prevents storage from being initialized while already in cluster mode, since the "config" database won't exist yet the first time we connect.

Instead, distinguish between `AphrontSchemaQueryException` (thrown on connection if the requested…