install_install.php is entirely separate from install_update.php and install_convert.php. Each of them perform this check in some form or another, so my goal here was to remove that duplication. Throwing an error would need to be done in install/index.php as the other three files are loaded as modules. Having such a check in index.php would not negate the need for the installation checks in the modules, however, as we would still need them to decide whether to list modules or not.