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

remoting/host/curtain_mode_mac.cc

Index: remoting/host/curtain_mode_mac.cc
diff --git a/remoting/host/curtain_mode_mac.cc b/remoting/host/curtain_mode_mac.cc
index 7a7b8c1f24f19d423af9649ed1ebca5114e4e74b..bf3118b88321f344f361462ce1ee1ab5cede8f2b 100644
--- a/remoting/host/curtain_mode_mac.cc
+++ b/remoting/host/curtain_mode_mac.cc
@@ -91,6 +91,7 @@ bool CurtainModeMac::ActivateCurtain() {
   // Try to install the switch-in handler. Do this before switching out the
   // current session so that the console session is not affected if it fails.
   if (!InstallEventHandler()) {
+    LOG(ERROR) << "Failed to install the switch-in handler.";
     return false;
   }