Fix misleading error message when only cluster database masters are configured

Summary:
Fixes T11446. We can raise the misleading error:

No valid databases are configured!

...when a valid master is configured but unreachable.

Instead, more carefully raise either "nothing is configured" or "nothing is reachable".

Test Plan: Configured only a master, artificially severed it, got "nothing is reachable" instead of "nothing is configured".

Reviewers: chad

Reviewed By: chad

Maniphest Tasks: T11446

Differential Revision: https://secure.phabricator.com/D16386