json.onError

In dvl.ajax.requester['normal' || 'cache'], can you replace the onError callback to call with 

``` javascript
normal:
  //...
  complete(xhr.responseText ? textStatus, null)    // complete(textStatus, null)
cache:
  //...
  cb(xhr.responseText ? textStatus, null);               // cb(textStatus, null)
```

It allows for response body capture, even though it's an error.

Best,
Barret
