Calls application and handles following cases:
            * catches `webob.HTTPException` errors.
            * catches unhandled exceptions, calls `handle_error` method
              and returns 500.
            * returns 404 if the app has returned None`.

def handle(self, env, data):
        '''
        Calls application and handles following cases:
            * catches `webob.HTTPException` errors.
            * catches unhandled exceptions, calls `handle_error` method
              and returns 500.
            * returns 404 if the app has returned None`.
        '''
        try:
            response = self.handler(env, data)
            if response is None:
                logger.debug('Application returned None '
                             'instead of Response object')
                response = HTTPNotFound()
        except HTTPException as e:
            response = e
        except Exception as e:
            self.handle_error(env)
            response = HTTPInternalServerError()
        return response