better error message, optionally exit if no file (#3934)

Project/Assets/ML-Agents/Examples/SharedAssets/Scripts/ModelOverrider.cs

     {
         const string k_CommandLineModelOverrideFlag = "--mlagents-override-model";
         const string k_CommandLineQuitAfterEpisodesFlag = "--mlagents-quit-after-episodes";
+        const string k_CommandLineQuitOnLoadFailure = "--mlagents-quit-on-load-failure";
 
         // The attached Agent
         Agent m_Agent;
         int m_MaxEpisodes;
 
         int m_NumSteps;
+
+        bool m_QuitOnLoadFailure;
 
         /// <summary>
         /// Get the asset path to use from the commandline arguments.
             var maxEpisodes = 0;
 
             var args = Environment.GetCommandLineArgs();
-            for (var i = 0; i < args.Length - 1; i++)
+            for (var i = 0; i < args.Length; i++)
             {
                 if (args[i] == k_CommandLineModelOverrideFlag && i < args.Length-2)
                 {
                 }
-                else if (args[i] == k_CommandLineQuitAfterEpisodesFlag)
+                else if (args[i] == k_CommandLineQuitAfterEpisodesFlag && i < args.Length-1)
+                }
+                else if (args[i] == k_CommandLineQuitOnLoadFailure)
+                {
+                    m_QuitOnLoadFailure = true;
                 }
             }
 
             var behaviorName = bp.BehaviorName;
 
             var nnModel = GetModelForBehaviorName(behaviorName);
-            Debug.Log($"Overriding behavior {behaviorName} for agent with model {nnModel?.name}");
+            if (nnModel == null && m_QuitOnLoadFailure)
+            {
+                Debug.Log(
+                    $"Didn't find a model for behaviorName {behaviorName}. Make " +
+                    $"sure the behaviorName is set correctly in the commandline " +
+                    $"and that the model file exists"
+                );
+                Application.Quit(1);
+            }
+            var modelName = nnModel != null ? nnModel.name : "<null>";
+            Debug.Log($"Overriding behavior {behaviorName} for agent with model {modelName}");
             // This might give a null model; that's better because we'll fall back to the Heuristic
             m_Agent.SetModel($"Override_{behaviorName}", nnModel);