Session PDO error handling

This adds the missing PDO error handling for the `PdoSessionHandler` as mentioned in https://github.com/symfony/symfony/issues/8095.

This hasn't been a problem until 2.3 (https://github.com/symfony/symfony/pull/7634 introduced the `InvalidArgumentException`), but should still be done in older versions (to make sure there's `PDOException`s rather than just `ErrorException`s).