install_database_errors() no longer returns rendered list of errors

install_database_errors() is used by core in both the SiteSettingsForm used to configure a site on installation as well as early in install_verify_database_settings() to report problems with the database if a database connection is already defined in settings.php. Previously, this function included a rendered summary of the errors in its return values. This rendered summary is only needed for the site settings form and unnecessarily adds a dependency on the render system for the early installer, so it has been moved into the SiteSettingsForm only. This change is unlikely to affect any code outside Drupal core.