When monitor command fails, raise it and not only log it

Otherwise the caller does not know that the command failed.

src/util.js

@@ -82,7 +82,7 @@ export function monitor(name, args, callback, on_close, system) {
                 if (e.name === "ConnectionClosed")
                     on_close(system);
                 else
-                    console.log(e);
+                    return Promise.reject(e);
             });
 }