Replace error throw with a log

UnitySDK/Assets/ML-Agents/Scripts/LearningBrain.cs

 #else
             if (agentInfos.Count > 0)
             {
-                throw new UnityAgentsException(string.Format(
+                Debug.LogError(string.Format(
                     "The brain {0} was set to inference mode but the Tensorflow library is not " +
                     "present in the Unity project.",
                     name));