throw ENOENT when file is not found

When trying to load a not-existing-file in node environment, currently the script does nothing, it simply hangs (in the browser xhr correctly throws 404).

This PR throws the ENOENT error raised by `fs.readFileSync` in require.text.js.
You mean:
```
onLoad.error = function(err) {
  onLoadNative.error(err);
};
```
? Yeah, that does it too.