Make friends leaderboard warn with Steam API is down

[tsa96]

Valve in their infinite wisdom/lack of engineers that give a shit/lack of serious competition from other platforms have a significant Steam outage every week, which stops filtering leaderboards by friends.

New backend responds with a 503 when this happens. Game should have a specific OnlineTimeStatus_t value for this, and display an appropriate error. See TODO in c_mom_leaderboards.cpp.

This is very low priority, and no reason to do until we do larger leaderboard reworks in the future. Blocked then, once we spec out of those changes, reference this issue/include it in a project board.

Tom note: 503 is probably a bad error code, we don't care about it showing up in logs/Sentry. Maybe just move to some 4xx unused by that endpoint).