Log to stderr if a monitor lock file exists

Reviewed By: ljw1004

Differential Revision: D34771406

fbshipit-source-id: 41f3ef0513c6ccb9c92c2cebced19aa87e88b029

hphp/hack/src/server/hhServerMonitor.ml

@@ -45,7 +45,10 @@ let monitor_daemon_main
      and handle requests. *)
   (if not (ServerArgs.check_mode options) then
     let lock_file = ServerFiles.lock_file www_root in
-    if not (Lock.grab lock_file) then Exit.exit Exit_status.No_error);
+    if not (Lock.grab lock_file) then (
+      Printf.eprintf "Monitor lock file already exists: %s\n%!" lock_file;
+      Exit.exit Exit_status.No_error
+    ));
 
   (* Daemon mode (should_detach): --daemon means the caller already spawned
      us in a new process, and it's now our responsibility to establish a logfile