better message for new model format (#5240)

Author: Chris Elion

com.unity.ml-agents/Runtime/Inference/BarracudaModelParamLoader.cs

@@ -66,13 +66,22 @@ public static IEnumerable<string> CheckModel(Model model, BrainParameters brainP
                     "Cannot use this model.");
                 return failedModelChecks;
             }
-            if (modelApiVersion != k_ApiVersion)
+            if (modelApiVersion < k_ApiVersion)
             {
                 failedModelChecks.Add(
                     $"Version of the trainer the model was trained with ({modelApiVersion}) " +
                     $"is not compatible with the Brain's version ({k_ApiVersion}).");
                 return failedModelChecks;
             }
+            if (modelApiVersion > k_ApiVersion)
+            {
+                failedModelChecks.Add(
+                    "Model was trained with a newer version of the trainer than is supported. " +
+                    "Either retrain with an older trainer, or update to a newer version of com.unity.ml-agents.\n" +
+                    $"Model version: {modelApiVersion}  Supported version: {k_ApiVersion}"
+                );
+                return failedModelChecks;
+            }
 
             var memorySize = (int)model.GetTensorByName(TensorNames.MemorySize)[0];
             if (memorySize == -1)