Fixed return types.

* `ExtendedPdo::connect` throws `\PDOException`.
* `ExtendedPdo::exec` returns affected rows.
* `ExtendedPdoInterface::connect` throws `\PDOException`
  (slash needed, no use).
* `PdoInterface::exec` returns affected rows.
* `PdoInterface::prepare` returns `\PDOStatement`
  (slash needed).
* `PdoInterface::query` returns `\PDOStatement`
  (slash needed, no use).