Calrify exceptional cases in ResponseTimeMiddleware

ResponseTimeMiddleware currently logs errors when `process_response` sees a request that hasn't been processed by `process_request`. However, according to the Django docs, this is a common situation caused by other middleware returning a response from `process_request` (e.g. a redirect).

See https://docs.djangoproject.com/en/dev/topics/http/middleware/#process-response.