Better handle service/sensor outages or failure

[hughbris]

I have worked around an unreliable hardware sensor (bike bell) failure by implementing "Faux Connection" mode. The app still handles outages and failures poorly for:

• GPS: we at least need some good GPS fixes for the app to be any use, but it will fail in situations like tunnels or some built-up areas - so outages OK, failures not
• Online status: offline map tiles, journey storage and modification (see also Delayed uploads and ability to update uploads #5) - the app should be able to function almost fully offline.