Improve warnings and exception if using unsupported combo

com.unity.ml-agents/Editor/BehaviorParametersEditor.cs

             if (brainParameters != null)
             {
                 var failedChecks = Inference.BarracudaModelParamLoader.CheckModel(
-                    barracudaModel, brainParameters, sensorComponents);
+                    barracudaModel, brainParameters, sensorComponents, behaviorParameters.behaviorType
+                );
                 foreach (var check in failedChecks)
                 {
                     if (check != null)

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

         /// </param>
         /// <param name="sensorComponents">Attached sensor components</param>
         /// <returns>The list the error messages of the checks that failed</returns>
-        public static IEnumerable<string> CheckModel(Model model, BrainParameters brainParameters, SensorComponent[] sensorComponents)
+        public static IEnumerable<string> CheckModel(Model model, BrainParameters brainParameters,
+            SensorComponent[] sensorComponents, BehaviorType behaviorType = BehaviorType.Default)
-                failedModelChecks.Add(
-                    "There is no model for this Brain, cannot run inference. " +
-                    "(But can still train)");
+                var errorMsg = "There is no model for this Brain; cannot run inference. ";
+                if (behaviorType == BehaviorType.InferenceOnly)
+                {
+                    errorMsg += "Either assign a model, or change to a different Behavior Type.";
+                }
+                else
+                {
+                    errorMsg += "(But can still train)";
+                }
+                failedModelChecks.Add(errorMsg);
                 return failedModelChecks;
             }
 

com.unity.ml-agents/Runtime/Policies/BehaviorParameters.cs

                 case BehaviorType.HeuristicOnly:
                     return new HeuristicPolicy(heuristic);
                 case BehaviorType.InferenceOnly:
+                {
+                    if (m_Model == null)
+                    {
+                        var behaviorType = BehaviorType.InferenceOnly.ToString();
+                        throw new UnityAgentsException(
+                            $"Can't use Behavior Type {behaviorType} without a model. " +
+                            "Either assign a model, or change to a different Behavior Type."
+                        );
+                    }
+                }
                 case BehaviorType.Default:
                     if (Academy.Instance.IsCommunicatorOn)
                     {

com.unity.ml-agents/Tests/Editor/BehaviorParameterTests.cs

+using NUnit.Framework;
+using UnityEngine;
+using MLAgents;
+using MLAgents.Policies;
+
+namespace MLAgents.Tests
+{
+    [TestFixture]
+    public class BehaviorParameterTests
+    {
+        static float[] DummyHeuristic()
+        {
+            return null;
+        }
+
+        [Test]
+        public void TestNoModelInferenceOnlyThrows()
+        {
+            var gameObj = new GameObject();
+            var bp = gameObj.AddComponent<BehaviorParameters>();
+            bp.behaviorType = BehaviorType.InferenceOnly;
+
+            Assert.Throws<UnityAgentsException>(() =>
+            {
+                bp.GeneratePolicy(DummyHeuristic);
+            });
+        }
+    }
+}