Fix missing possibility of assigning a CSS-class

Due to the missing pattern "%s" in the property
  protected $messageOpenFormat = '<ul%s><li>';
it was not possible to set a class in ViewScript via
  $this->formElementErrors()->setAttributes(array('class' => 'my-errorlist-class'));

If you do nothing the default value for css-class "help-block" is kept.
Merge pull request #70 from fg-ok/patch-1

Fix missing possibility of assigning a CSS-class
Update TwbBundleFormRow.php