Add helpful error message for null steps

src/Runner.Worker/ActionManifestManager.cs

@@ -419,6 +419,7 @@ namespace GitHub.Runner.Worker
                 {
                     if (stepsLoaded == null)
                     {
+                        // TODO: Add a more helpful error message + including file name, etc. to show user that it's because of their yaml file
                         throw new ArgumentNullException($"No steps provided.");
                     }
                     else