This way, if someone tries to install MediaWiki via
either web installer or install.php maintenance script,
after obtaining from git, they get some useful information
on what to do.
Put the checks alongside the php version check, as
stuff installed via composer (mustache / lightncandy)
is used for NoLocalSettings.php and install.php
attempts to use logging stuff.
Also tried to make PHPVersionError look slightly nicer,
with some <h2> elements and more padding for the <p>
elements.
and centralized this code in one place, as much as possible,
for improved ease of maintenance.
Bug: T90438
Bug: T88951
Change-Id: Iae4eb42c4266dbe9213c5de8a96fccfbeaa9acb0
9880d5b60b