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

A follow-up for #110.

/to @bajtos 
/cc @raymondfeng 
Consider this handler:

```
function(err, req, res, next) { next(err); }
```

The current implementation will skip the error handler provided by the REST adapter and pass the error to the app where the adapter is mounted on.

Is that our intention?

The alternative is to pass `restErrorHandler` to the `next` argument of the custom handler.

Also the current implementation is tracking an changes of `options.handler`, I am not sure if it is a good thing. An alternative version:

```js
RestAdapter.errorHandler = function(options) {
   options = options || {};
   if (options.handler) return options.handler;
   return function restErrorHandler(err, req, res, next) {
     // etc.
   };
}
```

Lastly, the line L309 is missing a semicolon.