Authorisation failure (401) masked by 502 from Nginx

[grahammcculloch]

Apparently if we make a POST request and the Node.js server responds with 401 because the user's session has expired, the Nginx server just responds with 502 to the client so the mobile app doesn't know that we need to log in again.

This results in some upload failures that confuse the user.

[grahammcculloch]

I'm going to try to mitigate this issue by checking the session whenever the app is foregrounded. If the session is about to expire, we'll return to the login page. Its unlikely that anyone will keep the app in the foreground for long enough for the session to expire while they're using the app in the foreground!