Display a nice exception message on ajax requests

Comparing #1 with #2

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But
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in Yii versions previous to 1.1.9 exceptions was handled differently during ajax request and displayed by [CApplication::displayException()](http://www.yiiframework.com/doc/api/1.1/CApplication#displayException-detail "displayException()"). This way the exception message displayed on ajax requests could not be customized.
 
 
The message for the CGridView delete request with YII_DEBUG set to true looked like
![old exception message on ajax request](http://i1107.photobucket.com/albums/h397/mdomba/yiierror-old.jpg "old exception message on ajax request")

In Yii version 1.1.9 the check for ajax request has been removed so now the exception message is handled by [CErrorHandler::errorAction](http://www.yiiframework.com/doc/api/1.1/CErrorHandler#errorAction-detail "errorAction") even during ajax requests.

This way the message can be customized for ajax requests.
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