Handle missing default.settings.php on installation

I recently forgot to copy default.settings.php to settings.php when installing Drupal. And when the installer alerted me to my oversight, I read too fast and renamed the file instead of copying it, resulting in a critical PHP error when I returned to the installer. I know it should never be missing, but since the installer presently requires manual user action on the file (see #322250: Automatically copy default.settings.php to settings.php on installation) it gets exposed to problems like that. Besides... it happened to me already. :)