Chutney should give better failure info when its Tor has --disable-module-dirauth

If Tor can't be a directory authority, you can't have a private network, so chutney won't work. But chutney doesn't notice, and it tries to run anyway, instead of telling you what the problem is.

This just consumed 20 minutes of my morning. Maybe we can fix it along with our other plans to give chutney a "will this network work" command.