Add logging to CHECK failure during curtain-mode on Mac.

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 cae8e704dfa1e202cc1380b841e2e830548603b6..0ac4aa5c2bdda50a61d5fde793c6de12c9cc2bf1 100644
--- a/remoting/host/curtain_mode_mac.cc
+++ b/remoting/host/curtain_mode_mac.cc
@@ -144,7 +144,10 @@ void SessionWatcher::ActivateCurtain() {
   // this, or how common it is, a crash report is useful in this case (note
   // that the connection would have to be refused in any case, so this is no
   // loss of functionality).
-  CHECK(session != nullptr);
+  CHECK(session != nullptr)
+      << "Error activating curtain-mode: "
+      << "CGSessionCopyCurrentDictionary() returned NULL. "
+      << "Logging out and back in should resolve this error.";
 
   const void* on_console = CFDictionaryGetValue(session,
                                                 kCGSessionOnConsoleKey);