Use our own error message when composer is not set up

Currently, when phpBB is installed from the repo without dependencies being set up, the error message is as follows:

PHP Fatal error: You have not set up composer dependencies. See http://getcomposer.org/. in /var/www/phpbb3/phpBB/includes/startup.php on line 176

We should include a more helpful error message and link to our own page.