Magento Enterprise – Errors Not Writing to var/report After Migration

It throws an exception, nothing more, nothing less.

That means, any code higher up the stack may catch it and do whatever it decides to. Typically it's one or more of the following:

1. Mage::logException() (this is where it gets added to exception.log)
2. show an error message to the user
3. just move on
4. throw another exception

My guess is on (3) in your case. Get a debugger installed on your development system, set a breakpoint at the Mage::throwException() line and you will find out. And I actually highly recommend looking at the core. It's one of the best resources to learn Magento development.

Update:

if you see a page with something like "An error occured, please contact the store owner" and a long number, this is what Magento shows you for uncaught exceptions. You will find the exception message and stack trace logged in var/report/HERE_COMES_THE_NUMBER

So the default behavior for an uncatched error (when developer mode is deactivated) is to create a report. That ultimate catch is handled in the Mage.php:

public static function init($code = '', $type = 'store', $options = array(), $modules = array())
{
    try {
        self::setRoot();
        self::$_app     = new Mage_Core_Model_App();
        self::_setIsInstalled($options);
        self::_setConfigModel($options);

        if (!empty($modules)) {
            self::$_app->initSpecified($code, $type, $options, $modules);
        } else {
            self::$_app->init($code, $type, $options);
        }
    } catch (Mage_Core_Model_Session_Exception $e) {
        header('Location: ' . self::getBaseUrl());
        die;
    } catch (Mage_Core_Model_Store_Exception $e) {
        require_once(self::getBaseDir() . DS . 'errors' . DS . '404.php');
        die;
    } catch (Exception $e) {
        self::printException($e);
        die;
    }
}

If you dig deeper into printException you can see that it decides if dev mode is activated or not. If it's activated it'll just output the exception and in production mode prepare the data and include errors/report.php.

public static function printException(Exception $e, $extra = '')
{
    if (self::$_isDeveloperMode) {
        print '<pre>';

        if (!empty($extra)) {
            print $extra . "\n\n";
        }

        print $e->getMessage() . "\n\n";
        print $e->getTraceAsString();
        print '</pre>';
    } else {

        $reportData = array(
            !empty($extra) ? $extra . "\n\n" : '' . $e->getMessage(),
            $e->getTraceAsString()
        );

        // retrieve server data
        if (isset($_SERVER)) {
            if (isset($_SERVER['REQUEST_URI'])) {
                $reportData['url'] = $_SERVER['REQUEST_URI'];
            }
            if (isset($_SERVER['SCRIPT_NAME'])) {
                $reportData['script_name'] = $_SERVER['SCRIPT_NAME'];
            }
        }

        // attempt to specify store as a skin
        try {
            $storeCode = self::app()->getStore()->getCode();
            $reportData['skin'] = $storeCode;
        }
        catch (Exception $e) {}

        require_once(self::getBaseDir() . DS . 'errors' . DS . 'report.php');
    }

    die();
}

errors/reports.php itself just calls the processor and stores the report:

require_once 'processor.php';

$processor = new Error_Processor();

if (isset($reportData) && is_array($reportData)) {
    $processor->saveReport($reportData);
}

$processor->processReport();

So, but where is the exception.log coming from? That's written when Mage::logException is called:

public static function logException(Exception $e)
{
    if (!self::getConfig()) {
        return;
    }
    $file = self::getStoreConfig('dev/log/exception_file');
    self::log("\n" . $e->__toString(), Zend_Log::ERR, $file);
}

An that static function is manually called in over 100 plcaes over the Magento Core, especially when an exception is catched, should be logged, but not thrown to the top of Mage::init to continue the current process.