Fix prepareRedirect in Mage_Core_Controller_Varien_Exception

Use case:
We create observer to event 'controller_action_predispatch' that check does user is logged in and not located in login page, than we create exception and prepare redirect to login page.

```php
$e = new Mage_Core_Controller_Varien_Exception();
$e->prepareRedirect('customer/account/login');

throw $exception;
```

PS: how I can correct create this exception (something like Mage::getModel())?