Adding exception object to the list of callback params

Optional., not breaking.

src/Loading/babylon.sceneLoader.ts

@@ -101,7 +101,7 @@
             SceneLoader._registeredPlugins.push(plugin);
         }
 
-        public static ImportMesh(meshesNames: any, rootUrl: string, sceneFilename: string, scene: Scene, onsuccess?: (meshes: AbstractMesh[], particleSystems: ParticleSystem[], skeletons: Skeleton[]) => void, progressCallBack?: () => void, onerror?: (scene: Scene, e: any) => void): void {
+        public static ImportMesh(meshesNames: any, rootUrl: string, sceneFilename: string, scene: Scene, onsuccess?: (meshes: AbstractMesh[], particleSystems: ParticleSystem[], skeletons: Skeleton[]) => void, progressCallBack?: () => void, onerror?: (scene: Scene, message: string, exception?: any) => void): void {
             if (sceneFilename.substr && sceneFilename.substr(0, 1) === "/") {
                 Tools.Error("Wrong sceneFilename parameter");
                 return;