Not sure if we should take into account corner cases like people using not TorLauncher but a system-wide tor. If so, we should retitle the ticket accordingly. In any case, what I'd like to avoid is a Tor Browser showing everything is okay but no tor is launched to get used for browsing.
The basic idea is to show failure on the about:tor page when Torbutton cannot connect to the proxy (i.e., to tor). We tested it by disabling Tor Launcher.