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

Resolves issue #2223. If `errorMessageCssClass` is not specified explicitly, it will fall back to the default, specified with `CHtml::$errorMessageCss`.

The user-specified `CHtml::$errorMessageCss` will now work on both ajax and normal errors. If overloaded in `CActiveForm`, the overloaded value will take precedence.