Display correct error message if some self checks could not be performed due network timeout to overwrite.cli.url

Tested on Nextcloud 31.0.4.
My setup: Nextcloud standalone in a debian machine behind a reverse proxy.
Initial situation: During an upgrade I always block incoming requests (except my admin machine) via firewall on the reverse proxy. After the update I look for the self check in /index.php/settings/admin/overview for errors and warnings. Only after resolving the issues I proceed with unblocking firewall traffic.

Nextcloud's self check makes a request to the URL configured in overwrite.cli.url (config.php) and looks for an HSTS header in the response. In my case this request ran into a timeout. Nextcloud displayed me then the error message that I found here: https://github.com/nextcloud/server/blob/56b9974c419f797d85b9e48d1402374e6cfacfb6/apps/settings/lib/SetupChecks/SecurityHeaders.php#L109

HSTS is correctly set up on my reverse proxy. But as the Nextcloud instance could not reach the proxy it showed me this warning. I expect Nextcloud to display me an error message like "Some tests could not be performed due to a network timeout to the configured public address <address> configured in the config.php"

Start in apps/settings/lib/SetupChecks/SecurityHeaders.php around line 109, then trace how the self check is shown in /index.php/settings/admin/overview. Reproduce the timeout against the address configured in overwrite.cli.url and compare it with a successful HSTS check. Done means the timeout receives a distinct, actionable error message identifying the configured public address.