Handle RouteNotFoundException

| Q             | A
| ------------- | ---
| Bug fix?      | [yes]
| New feature?  | [no]
| BC breaks?    | [no]
| Deprecations? | [no]
| Tests pass?   | [yes]
| Fixed tickets | See https://groups.google.com/forum/#!topic/symfony-cmf-devs/o8l0qDOoH8M
| License       | MIT

When a menu node references a route, this route may not exist anymore, leading to a ``RouteNotFoundException``.

This PR handle this by logging the exception and by returning ``null`` or an empty menu node (depending on the configuration).

If there is no error, ``$options['uri']`` is set and ``$options['route']`` is unset.

@dbu I prefered to directly try/catch the route generation to be able to return an empty item.