EZP-23516: fix generation for ez_legacy routes using url twig function

When trying to generate a route for legacy modules (languages switcher), it raised an error like 
```
An exception has been thrown during the rendering of a template ("When generating an eZ Publish legacy fallback route, "uri" parameter must be provided.") in EzConsultingBundle:Language:switcher.html.twig at line 17.
```
This modification solve issue https://jira.ez.no/browse/EZP-23516 
and generate the good url for legacy module using url twig function
```
{{ url( routeRef ) }}
```

Wrong exception :-).

```php
use RuntimeException;
```