Update CActiveForm.php

If 'errorMessageCssClass' is not specified explicitly, it will fall back to the default, specified with CHtml::$errorMessageCss
Update CActiveForm.php

Added version number to comment, minor styling fixes
Update CHANGELOG
Merge pull request #2231 from opus-online/activeform-fix-2223

Fix for #2223 - `CActiveForm::error` does not respect `CHtml::$errorMessageCss`