Write an error message to the debug log when the host is shutting down because of a policy setting.

remoting/host/curtain_mode_linux.cc

 // Copyright (c) 2012 The Chromium Authors. All rights reserved.
 // Use of this source code is governed by a BSD-style license that can be
 // found in the LICENSE file.
 
 #include "remoting/host/curtain_mode.h"
 
 #include <X11/extensions/XInput.h>
 
 #include "base/callback.h"
 #include "base/logging.h"
@@ skipping 77 matching lines @@
   XFreeDeviceList(devices);
   XCloseDisplay(display);
   return found_xvfb_mouse && found_xvfb_keyboard && !found_other_devices;
 }
 
 void CurtainModeLinux::SetActivated(bool activated) {
   // We can't curtain the session in run-time in Linux.
   // Either the session is running on Xvfb (i.e. always curtained), or it is
   // attached to the physical console (i.e. impossible to curtain).
   if (activated && !IsXvfbSession()) {
+    LOG(ERROR) << "Curtain-mode is not supported when running on non-Xvfb "
+                  "X server";
     on_error_.Run();
   }
 }
 
 // static
 scoped_ptr<CurtainMode> CurtainMode::Create(
     const base::Closure& on_session_activate,
     const base::Closure& on_error) {
   return scoped_ptr<CurtainMode>(
       new CurtainModeLinux(on_session_activate, on_error));
 }
 
 }  // namespace remoting