Return validation errors with in the response (blueimp)

The validation errors are ignored in most controllers. This commit will pass the validation error to the response with the blueimp frontend. Works with single-request uploads too, but had to extend the response object for multi-dimensional array support.

Also fixes a small mistake accessing multiple files in the FileBag.

Didn't touch the documentation, but it should mention to use the
````php
$request->addToOffset($value, $offset,...);
```` 
instead of simple array access, in case of single-request file uploads.