N°4698 setup/phpinfo.php : when no valid iTop installation, display a comprehensive message instead of an exception

[piRGoif]

When calling the setup/phpinfo.php page on a iTop that has not been installed yet, you're getting this exception:

Indeed due to security reasons this page is only accessible to admin users, and this authentication needs a valid iTop instance... In such a scenario, there is still the setup/phpcheck.php page (thanks @BenGrenoble !) that can be used though, but you'll need to know it...

This PR adds a comprehensive message when calling the page in such scenario:

Also a comment was added to the file beginning to recall the existence of setup/phpcheck.php