improved makeCallback arg checking

Author: bjouhier

lib/fs.js

@@ -91,9 +91,12 @@ function maybeCallback(cb) {
 // for callbacks that are passed to the binding layer, callbacks that are
 // invoked from JS already run in the proper scope.
 function makeCallback(cb) {
-  if (!util.isFunction(cb)) {
+  if (util.ifNullOrUndefined(cb)) {
     return rethrow();
   }
+  if (!util.isFunction(cb)) {
+    throw new Error('callback must be a function');
+  }
 
   return function() {
     return cb.apply(null, arguments);