New warning if trips.shape_id references an unknown shapes.shape_id

[AntoineAugusti]

A warning is not raised at the moment. This creates confusion when trying to visualize shapes: a file exists but it's only possible to visualize stops.

https://github.com/etalab/transport-site/issues/2548

Sample GTFS with this problem.

gtfs_stan.zip

[fchabouis]

closed by #148