If JACK can't start and isn't running, launch fails badly

[bgribble]

I see this because I switch JACK to different interfaces all the time and sometimes the last interface JACK used is not available (USB device disconnected for example).

Normally, if you start MFP and JACK isn't running, it will start the server. But if the current server config doesn't work, the startup will fall over in a fairly ugly way and require C-C to exit.