MXS-1929: Allow startup with no services

MaxScale can now be started with an empty configuration file and services
can be created at runtime. Filters cannot yet be created at runtime so
complete runtime creation of configurations is not yet possible.

server/core/service.cc

@@ -650,7 +650,8 @@ int service_launch_all()
 
         ptr = ptr->next;
     }
-    return error ? 0 : n;
+
+    return error ? -1 : n;
 }
 
 bool serviceStop(SERVICE *service)