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

Index: remoting/host/curtain_mode_linux.cc
diff --git a/remoting/host/curtain_mode_linux.cc b/remoting/host/curtain_mode_linux.cc
index 61a3d9780450829ade043b3a8365512bdb7570c1..34b4db0ff174437b6dbc6d7f8c4b1d9fd3d2f9d8 100644
--- a/remoting/host/curtain_mode_linux.cc
+++ b/remoting/host/curtain_mode_linux.cc
@@ -95,6 +95,8 @@ void CurtainModeLinux::SetActivated(bool activated) {
   // 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();
   }
 }