fixed admin_messages in frontend, fixes #940

[sprankhub]

Following up on #940 here. The distinction between getHtml for the global_messages and getGroupedHtml for the messages is quite old:

https://github.com/nexcess/magento-turpentine/blame/44d7b7627b2656ae87aa29b5a6a52e4e774fd33d/app/code/community/Nexcessnet/Turpentine/Block/Core/Messages.php#L319-L327

It should be absolutely okay to always use getGroupedHtml:

• for the messages, getGroupedHtml is already used, so nothing changes for this block
• the global_messages block is always called via <?php echo $this->getChildHtml('global_messages') ?>
• Mage_Core_Block_Abstract::getChildHtml calls Mage_Core_Block_Abstract::_getChildHtml
• Mage_Core_Block_Abstract::_getChildHtml calls Mage_Core_Block_Abstract::toHtml
• Mage_Core_Block_Abstract::toHtml calls Mage_Core_Block_Messages::_toHtml
• Mage_Core_Block_Messages::_toHtml always uses Mage_Core_Block_Messages::getGroupedHtml - hence, we can use it as well

[sprankhub]

Any idea when this can be merged @miguelbalparda?