Add a specialized cache for storing "has setup ever worked?"

Summary:
Ref T11613. In D16503/T11598 I refined the setup flow to improve messaging for early-stage setup issues, but failed to fully untangle things.

We sometimes still try to access a cache which uses configuration before we build configuration, which causes an error.

Instead, store "are we in flight / has setup ever worked?" in a separate cache which doesn't use the cache namespace. This stops us from trying to read config before building config.

Test Plan:
Hit bad extension error with a fake extension, got a proper setup help page:

{F1812803}

Solved the error, reloaded, broke things again, got a "friendly" page:

{F1812805}

Reviewers: chad

Reviewed By: chad

Maniphest Tasks: T11613

Differential Revision: https://secure.phabricator.com/D16542