What problem in GTFS datasets does this new rule address? Please describe.
The spec in stops.stop_lat and stops.stop_lon mentions (source):

Conditionally Required:

• Required for locations which are stops (location_type=0), stations (location_type=1) or entrances/exits (location_type=2).
• Optional for locations which are generic nodes (location_type=3) or boarding areas (location_type=4).

This check is missing from the validator.

Describe the new validation rule
For each line in stops.txt, if either stop_lat or stop_lon (or both) are missing AND location_type = 0, 1 or 2, trigger a missing_stop_coordinates notice.

Error vs warning
Error