ErrorListener: Fixed wrong parameter being passed to the RuntimeException

Fixed wrong exception throwing

**PHP Error**

> Fatal error:  Wrong parameters for Exception([string $exception [, long $code [, Exception $previous = NULL]]]) in 
> \lib\Gitlab\HttpClient\Listener\ErrorListener.php</b> on line 60

**Request that throwed the error**

```php
// Gitlab throwed an error because the source and target branch are the same
$project->createMergeRequest("samebranchname", "samebranchname", "Some text");
```