[Question] Exception with Custom Attributes

Dear all,

first of all: thanks a lot for this awesome package! I really love it - keep your work up :)

However, I have a question regarding the Exception Handling, specifically how to add custom fields to exceptions. I would like to make the exceptions thrown JsonAPI-valid ([JSON API Errors](http://jsonapi.org/format/#errors)). I have googled a lot on this problem and tracked down a lot of issues on this project site as well. However, I was not able to solve my problem.

First of all, I have adapted the `api.php` config file, specifically the `errorFormat` to meet my demands.

Then I have created an `ApiBaseException` extending the `Symfony\Component\HttpKernel\Exception\HttpException` as described in the Wiki Pages ([Errors and Error Responses](https://github.com/dingo/api/wiki/Errors-And-Error-Responses))

The constructor for this Exception looks like this:

``` php
public function __construct(..., $title = null, $meta = []) {
parent::__construct(..., ..., ...);
$this->title = $title;
$this->meta = $meta;
}
```

because i would like to add the fields `title` and `meta` to my JSON Exception. For example, the result should look like this:

``` json
{ "errors" : {
"message" : "The Element could not be stored to the database!",
"code" : "INTERNAL CODE",
"status" : 500,
"title" : "ELEMENT_SAVE_ERROR",
"meta" : {
"more" : "information",
"further" : "data"
}
} }
```

However, if I throw the exception like this `throw new ApiBaseException(..., ..., ..., "ELEMENT_SAVE_ERROR", []);` within the controller this further information (e.g., `title` and `meta`) is not displayed.

It looks to me, that all information (except the default values provided in the original `api.php` config file) are omitted. How can I add the additional fields to the JSON Response for exceptions?

I would highly appreciate your answer or contributions to solve my problem. Please let me know, if you need any other information.

Kind regards,

Johannes