Update GLTFLoader.js

Error constructor takes one argument. Needs to be a concatenated string. Probably a holdover from `console.error` usage

examples/js/loaders/GLTFLoader.js

@@ -2064,7 +2064,7 @@ THREE.GLTFLoader = ( function () {
 
 					} else {
 
-						throw new Error( 'THREE.GLTFLoader: Primitive mode unsupported: ', primitive.mode );
+						throw new Error( 'THREE.GLTFLoader: Primitive mode unsupported: ' + primitive.mode );
 
 					}