<?php
namespace TwbBundle\Form\View\Helper;
class TwbBundleFormElementErrors extends \Zend\Form\View\Helper\FormElementErrors{
	protected $messageOpenFormat = '<ul class="help-block"><li>';
	protected $messageCloseString = '</li></ul>';
	protected $messageSeparatorString = '</li><li>';
}
<?php
namespace TwbBundle\Form\View\Helper;
class TwbBundleFormElementErrors extends \Zend\Form\View\Helper\FormElementErrors{
    protected $attributes = array(
        'class' => 'help-block'
    );
}

Fix missing possibility of assigning a CSS-class
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.