core-bundle/src/Resources/contao/library/Contao/Message.php
public static function generate($strScope=TL_MODE)
	{
		$strMessages = static::generateUnwrapped($strScope);

		if ($strMessages != '')
		{
			$strMessages = '<div class="tl_message">' . $strMessages . '</div>';
		}

		return $strMessages;
	}
public static function generate($strScope=TL_MODE)
	{
		$strMessages = static::generateUnwrapped($strScope);

		if ($strMessages != '')
		{
			$strMessages = '<div class="tl_message">' . $strMessages . '</div>';
		}

		return $strMessages;
	}
Return the messages with a wrapping container as HTML

@param string $strScope An optional message scope

@return string The messages HTML markup