R1:f82adbf0c8ff

Add a friendlier "in flight" error page

Summary:
Ref T11589. When we hit a fatal setup issue (essentially always a connection failure) //after// we've already survived them on at least one request, we can be pretty sure a server went down and that the problem is not a setup/configuration issue.

In this case, show a friendlier error page instead of the fairly detailed technical one.

Test Plan:
- Broke MySQL config.
- Restarted Apache.
- Got the "admin/setup" error page:

{F1803268}

- Fixed the…