Import should fail if some of the required fields (as marked in the gtfs spec) are missing?

[rmkujala]

Errors can relatively easily propagate after the import.

In case of problems, one could give the import_gtfs function separate keyword parameters that would let some missing data go through without failing.

Then, e.g. with a flag ("interpolate_missing_times_in_stop_times") would try to be smart, and interpolate any missing departure_time / arrival_time data.