The handling of database settings and/or connection problems is made more friendly.

For databases other then the ones that are supported by Drupal 8 they need to add support for:

1. There is a new protected method \Drupal\Core\Database\Connection::getSQLState(). This method extracts the SQLSTATE error from the PDOException. This method may need to be overriden.
2. The opening of a PDO-connection for the by core supported databases have now exception handeling for a "Database Not Found Exception" and for a "Database Access Denied Exception". The code example for MySQL is:

     try {
       $pdo = new \PDO($dsn, $connection_options['username'], $connection_options['password'], $connection_options['pdo']);
     }
     catch (\PDOException $e) {
       if ($e->getCode() == static::DATABASE_NOT_FOUND) {
         throw new DatabaseNotFoundException($e->getMessage(), $e->getCode(), $e);
       }
       if ($e->getCode() == static::ACCESS_DENIED) {
         throw new DatabaseAccessDeniedException($e->getMessage(), $e->getCode(), $e);
       }
       throw $e;
     }