The class_exists check has been added in 4f29f58a5e (L1R28) to prevent a side effect with a PECL extension.
However if an autoloader such as the Composer one is loaded, it will try to require this `exceptions.php` file again:
```
'HttpException' => $vendorDir . '/pear-pear.cakephp.org/CakePHP/Cake/Error/exceptions.php',
```
this will result in a "Fatal error: Cannot redeclare class CakeBaseException"