throw exception if the base path node does not exist

| Q             | A
| ------------- | ---
| Bug fix?      | yes
| New feature?  | no
| BC breaks?    | no
| Deprecations? | no
| Tests pass?   | yes
| Fixed tickets | https://github.com/symfony-cmf/RoutingAutoBundle/issues/129
| License       | MIT
| Doc PR        | -