Method displayWithErrorHandling should not wrap all exceptions.

Hello,

I'm developing an application using Kohana framework and I've found it impossible to track an exact cause of an exception because Twig is wrapping it inside `Twig_Error_Runtime`.
Basically it prevents Kohana error handler from displaying any useful backtrace information I need to track the issue.

I believe that any exception that is not caused by Twig should not be thrown as `Twig_Error_Runtime`.