Homestyx hydra

R1:bc4187d70949

When we "discover" new fatal setup issues, stop serving traffic

Summary:
Ref T10759. We may "discover" the presence of a fatal setup error later, after starting Phabricator.

This can happen in a few ways, but most are unlikely. The one I'm immediately concerned about is:

- Phabricator starts up during a disaster with some databases unreachable.
- We start with warnings (unreachable databases are generally not fatal, since it's OK for some subset of hosts to be down in replicated/partitioned setups).
Repository: R1 hydra
Commit Date: Nov 21 2016