Station_status or vehicle_status should be required #196

Context

Flagged by @futuretap in MobilityData/gbfs-validator#216

Some feeds have no station_status.json and no vehicle_status.json endpoint. For example: https://opendata.agglo-larochelle.fr/sites/default/files/dataset/2dc/df9ac-ca89-4e3f-af7e-5d75f255ee5e/gbfs.json

However, a system of shared vehicles is either dock-based, or free-floating, or hybrid, and the spec indicates:

• station_status.json: REQUIRED of systems utilizing docks.
• vehicle_status.json: REQUIRED for free floating (dockless) vehicles.

Proposed Solution

The validator should return "invalid feed" if station_status.json and vehicle_status.json are both missing.

cc @emmambd