Lower the priority of the exception logging

Fixes #752

I'd like this to be lower, `Symfony\Component\HttpKernel\EventListener\ExceptionListener` has this functionality at [-128](https://github.com/webfactory/symfony/blob/6b64c03e4a92d071263b3c5c765de166106f2d2e/src/Symfony/Component/HttpKernel/EventListener/ExceptionListener.php#L84), but the default priority for handlers added with `Silex\Application::error` fire at -8, so I thought there would be fewer wtfs if we stay above that?