function ms_not_installed should set status header to 500

[dathomas21]

The function ms_not_installed() does not set the status header 500, stating that there was some kind of error on the page. This is essentially important because services like Varnish would cache the page since the status header is 200 and not 500, like it should be. The best way to reproduce this issue is to set up a WP environment using multisite, after you have the site fully set up, set a query limit on the DB user to only be able to execute 1 query an hour and you should be able to reproduce this issue.

Don't worry, my PROD environment does not have this actual DB user limitation, the issue that happened on my PROD environment was that the DB crashed after WP initially connected to the DB

[SergeyBiryukov]

Duplicate of #30002.