Output a warning for the sessionctl run mode

The sessionctl binary is no longer supported in many fuchsia configurations.

Change-Id: If2762ef8877f67796524d5ed7dd76ff0f297a06b
Testing: manual
diff --git a/src/lib.rs b/src/lib.rs
index 2699443..e08e20f 100644
--- a/src/lib.rs
+++ b/src/lib.rs
@@ -426,7 +426,10 @@
 
     match options.run_mode {
         RunMode::Tiles => runner_args.push(&tiles_arg),
-        RunMode::SessionCtl => runner_args.push(&session_ctl_arg),
+        RunMode::SessionCtl => {
+            println!("***** Warning: sessionctl is no longer supported in many Fuchsia configurations. *****");
+            runner_args.push(&session_ctl_arg)
+        }
         RunMode::Run => runner_args.push(&run_arg),
     }