When a GTFS URL points to a folder the resulting dataset file is invalid

[alaurenz]

There is still a case where if the GTFS link provided points to a folder (rather than a file) it will result in publishing an invalid file to the dataset record on data.bts.gov.

For example, if the URL is "https://metro.kingcounty.gov/GTFS/" this would result in an invalid file uploaded to the corresponding dataset on data.bts.gov.