This PR was merged into the 4.4 branch.

Discussion
----------

Add ErrorHandler component

| Q             | A
| ------------- | ---
| Branch?       | 4.4
| Bug fix?      | no
| New feature?  | yes
| BC breaks?    | no
| Deprecations? | yes
| Tests pass?   | no
| Fixed tickets | #25905, #26448
| License       | MIT
| Doc PR        | TODO

Mainly for API-based apps that don't require TwigBundle to get the correct exception response according to the request format (aka `_format` attribute).

![exception_response](https://user-images.githubusercontent.com/2028198/55509651-713dc700-562a-11e9-8b98-bef3b0229397.gif)

:heavy_check_mark: [RFC7807](https://tools.ietf.org/html/rfc7807) compliant for JSON and XML formats.

---

This introduce a new `ErrorRenderer` service that render a `FlattenException` into a given format:
```php
use Symfony\Component\ErrorHandler\ErrorRenderer\ErrorRenderer;
use Symfony\Component\ErrorHandler\ErrorRenderer\HtmlErrorRenderer;
use Symfony\Component\ErrorHandler\ErrorRenderer\JsonErrorRenderer;

$renderers = [
    new HtmlErrorRenderer(),
    new JsonErrorRenderer(),
    // ...
];
$errorRenderer = new ErrorRenderer($renderers);

return new Response(
    $errorRenderer->render($exception, $request->getRequestFormat()),
    $exception->getStatusCode(),
    $exception->getHeaders()
);
```

The built-in error renderers are:

| Format | Class |
| --- | --- |
| html | HtmlErrorRenderer |
| json | JsonErrorRenderer |
| xml, atom | XmlErrorRenderer |
| txt | TxtErrorRenderer |

And you can add your own error renderer by implementing the `ErrorRendererInterface` and tagging it with `error_handler.renderer` in your service definition.

Creating your own error renderer for a built-in format will end up replacing the related built-in error renderer.

Demo: https://github.com/yceruto/error-handler-app ([add custom error renderer](yceruto/error-handler-app@06fc647))

Commits
-------

7057244 Added ErrorHandler component