Add message for shader not found.

Scripts/Format/VRMImporter.cs

@@ -91,6 +91,7 @@ namespace VRM
                 var shader = Shader.Find(shaderName);
                 if (shader == null)
                 {
+                    Debug.LogErrorFormat("shader {0} not found. set Assets/VRM/Shaders/VRMShaders to Edit - project setting - Graphics - preloaded shaders", shaderName);
                     return fallback(ctx, i);
                 }
                 else