Add an action to show *error* pages in kernel.debug mode
feature #12096 Add an action to show *error* pages in kernel.debug mode (mpdude)

This PR was squashed before being merged into the 2.6-dev branch (closes #12096).

Discussion
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Add an action to show *error* pages in kernel.debug mode

| Q             | A
| ------------- | ---
| Bug fix?      | no
| New feature?  | yes
| BC breaks?    | no
| Deprecations? | no
| Tests pass?   | yes
| Fixed tickets | #7446, #1486, #11327
| License       | MIT
| Doc PR        | symfony/symfony-docs#4293

See #7446 for the initial reasoning. In short, add to your `routing_development.yml` file the following

```yaml
_errors:
    resource: "@TwigBundle/Resources/config/routing/errors.xml"
    prefix:   /_error
```

Then you can use `http://localhost/app_dev.php/_error/xxx` to preview the HTML *error* page that the default `ExceptionController` (from `TwigBundle`) would pick for the XXX status code.

You can also use `http://localhost/app_dev.php/_error/xxx.{some_format}` to show error pages for other formats than HTML, most notably `txt`.

Note that the status code will be 500 for all exceptions [that do not implement `HttpExceptionInterface`](https://github.com/symfony/symfony/blob/master/src/Symfony/Component/Debug/Exception/FlattenException.php#L47).

##### Want to test with a custom exception?

~~Folks might want to display (part of) the exception even on error pages and thus need to work with generic (own) exceptions.~~

~~They could write an arbitrary controller and throw their exception there. By default, the `ExceptionController` would be used to handle this, only that it would not show *error* pages in `kernel.debug` mode.~~

~~Thus, a simple public setter to change the `debug` flag after construction could help. Do we want to add that as well?~~

If you want to test error pages with your own exceptions,
* create a subclass of `ExceptionController`
* set the protected `debug` flag to false
* register it as twig.exception_controller in the config
* throw your custom exception from any controller.

That should give you the error page also in `kernel.debug` mode.

##### To-Do
- [x] Update docs
- [x] Add route in symfony/symfony-default

Commits
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66ed177 Add an action to show *error* pages in kernel.debug mode