Fixed bug in LocalStoreClearMiddleware.process_exception

Hi there. This bug caused our production site being down so I decided to notify you about it.

According to [Django documentation on middleware methods](https://docs.djangoproject.com/en/dev/topics/http/middleware/#process_exception) the _process_exception_ method should not re-raise exception ever. Re-raising exception causes an error in middleware calls chain.

TL;DR: _process_exception_ should not raise any exceptions or return anything except of response (if any).