fixed getting cause from exceptions

The formatters are accessing the `$cause` property of any exception as if it was defined and public. 
This can lead to fatal errors when an Exception class is caught that actually has a property named `$cause` and it is not public f.e.:

```php
PHP Fatal error:  Cannot access protected property SomeException::$cause in "..."/vendor/phpspec/phpspec2/src/PHPSpec2/Formatter/ProgressFormatter.php on line 115
```