Summary:
If the system can't load, we often have good error screens, but this check blocks them because the Config wasn't loaded.

Just skip the preconnect header if config is broken - probably most of the site isn't working anyway.

Fixes T16173.

Test Plan: remove php-curl, php-mbstring and php-mysql, restart server, see nicer error message with references to `apt-get` and `dnf`. It might be enough to remove just one of them.

Reviewers: aklapper, luca.itro, O1 Blessed Committers

Reviewed By: aklapper, O1 Blessed Committers

Subscribers: tobiaswiese, valerio.bozzolan, Matthew, Cigaryno

Maniphest Tasks: T16173

Differential Revision: https://we.phorge.it/D26576