die on failed startup

when init, start were made async, the eventloop would continue to run after an exception

jupyterhub/app.py

@@ -1011,8 +1011,12 @@ class JupyterHub(Application):
     
     @gen.coroutine
     def launch_instance_async(self, argv=None):
+        try:
             yield self.initialize(argv)
             yield self.start()
+        except Exception as e:
+            self.log.exception("")
+            self.exit(1)
     
     @classmethod
     def launch_instance(cls, argv=None):