module: Set the error.code to 'MODULE_NOT_FOUND' when a bad path is given to require.

Fixes #2358.

lib/module.js

@@ -331,7 +331,9 @@ Module._resolveFilename = function(request, parent) {
 
   var filename = Module._findPath(request, paths);
   if (!filename) {
-    throw new Error("Cannot find module '" + request + "'");
+    var err = new Error("Cannot find module '" + request + "'");
+    err.code = 'MODULE_NOT_FOUND';
+    throw err;
   }
   id = filename;
   return [id, filename];