Startup errors

 - update server to send startup errors in an argument of the /exited message
 - update GUI to report server errors, as well as inability to listen on the OSC port
 - this covers several cases with more useful error messages: scsynth still running, ruby still running, Sonic Pi already running, scsynth failed to start
 - note that parenting the QProcess to `this` apparently keeps us from shutting down scsynth cleanly on Windows, so that's not an improvement
 - startup code should be rewritten for 2.2, especially the sleep(1) in the GUI thread (yuck)