proper return code;

--- a/src/Pure/Tools/ml_process.scala	Wed Apr 06 14:02:12 2016 +0200
+++ b/src/Pure/Tools/ml_process.scala	Wed Apr 06 14:08:57 2016 +0200
@@ -140,7 +140,8 @@
     val more_args = getopts(args)
     if (args.isEmpty || !more_args.isEmpty) getopts.usage()
 
-    ML_Process(options, logic = logic, args = eval_args, dirs = dirs, modes = modes).
+    val rc = ML_Process(options, logic = logic, args = eval_args, dirs = dirs, modes = modes).
       result().print_stdout.rc
+    sys.exit(rc)
   })
 }