… need to set kernel.debug=false (mpdude)

This PR was submitted for the master branch but it was merged into the 2.3 branch instead (closes #3577).

Discussion
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Development of custom error pages is impractical if you need to set kernel.debug=false

Also see
symfony/symfony#7446 and symfony/symfony#1486

Commits
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6aa90e0 Use WebfactoryExceptionsBundle instead of webfactory/exceptions-bundle
2fee263 Update error_pages.rst
435019c Point out the difference between error and exception pages.
2194607 Add a pointer to webfactory/exceptions-bundle which helps with development of custom error pages.

…de (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
-------

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