Changed errorHandler() to honor options.remoting.errorHandler.handler

A follow-up for #110.

/to @bajtos 
/cc @raymondfeng 
If I interpret our discussion in #102 correctly, then you @ritch need to rework this PR so that the following usage is supported:

```js
options: {
  errorHandler: function(err, req, res, next) {
    console.error(err);
    next(); // to use default error handler
  }
}
```