R1:f31975f7a3be

Don't emit Content-Security-Policy when returning a response during preflight setup checks

Summary:
Ref T4340. See <https://discourse.phabricator-community.org/t/core-exception-during-installation/1193/8>.

If we return a response very early during setup, we may not be able to read from the environment yet. Just decline to build a "Content-Security-Policy" header in these cases.

Test Plan:
- Faked a preflight error (e.g., safe_mode enabled), restarted apache.
- Before patch: environment error while…