[HttpKernel] exception listener, allow easier overloading of the default behavior

Hi Fabien,

This commit you wrote about a month ago helped me to overload the 'logException' method:
https://github.com/symfony/symfony/commit/1a6c9b31438e45180001957bb64f97d1cf54ca4f

Because the logger property was private I could not access it from the subclass. This PR changes both properties: controller and logger from private to public members. So they can be used from 'logException'.