When bridges are enabled, refuse to run without bridge support.

Right now if you configure bridges.enable=true, and start an Arti that was built without bridge support, it will merrily attempt a direct connection to the network.

That's probably undesirable: instead we should say "You asked for bridges, but I don't support that. Since I can't do what you wanted here, I will exit." Otherwise people who asked for censorship-resistant connections would get direct ones, which could put them at risk.

cc @Diziet