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 MySQL config.
• Loaded any page, to put us "in flight".
• Broke MySQL config.
• Loaded any page.
• Got the friendly "in flight" error page:

{F1803271}

If you want to design this better, easiest way to get to it is:

• Set mysql.port to 9999 in conf/local/local.json.
• Reload any page while already running (don't restart).

Reviewers: chad

Reviewed By: chad

Maniphest Tasks: T11589

Differential Revision: https://secure.phabricator.com/D16503