Added 502 cache_miss handler

Hi, Thanks for publishing your work.

This is a small tweak to your example nginx config. On my setup a memcached outage/stoppage sends a  crude 502 to the client. This can be handled in the same manner as a 404 @cache_miss. Assuming the backend django process is still responsive.

It might not be the most ideal handler heavy loads or extended outages but it's probably a sensible default.