Show meaningful error if ModelLoaderRegistry is used before the missing model is initialized.

src/main/java/net/minecraftforge/client/model/ModelLoaderRegistry.java

@@ -192,6 +192,10 @@ public class ModelLoaderRegistry
 
     public static IModel getMissingModel()
     {
+        if(ModelLoader.VanillaLoader.INSTANCE.getLoader() == null)
+        {
+            throw new IllegalStateException("Using ModelLoaderRegistry too early.");
+        }
         return ModelLoader.VanillaLoader.INSTANCE.getLoader().getMissingModel();
     }