Fix missing possibility of assigning a CSS-class

Due to the missing pattern "%s" in the property
```php
  protected $messageOpenFormat = '<ul%s><li>';
``` 
it was not possible to set a class in ViewScript via
```php
  $this->formElementErrors()->setAttributes(array('class' => 'my-errorlist-class'));
``` 

Now the behaviour is still: You do nothing the default value for css-class "help-block" is kept, but if you want to you are able to set/add own css-classes.