Added a Twig filter to pull the exception class name from an exception

I was trying to transform my error view PHP template to a twig template. Doing this I was not really able to replace the get_class($exception) part of it. First I tried to make an own version of the ExceptionStrategy which puts more variables into the ViewModel but the implementation didn't feel clean and maybe a filter is better.

If you are liking this addition, should I also make application/error/index.twig and application/error/404.twig examples in this pull-request?