Add check for shapes.txt and no shapes_dist_traveled

This adds a warning message to the import process if
both a shapes.txt and no shapes_dist_traveled are available
since length for routes and modes will not be available.

Note: In developing this, no gtfs files with a shape_dist_traveled,
but no shapes.txt file were able to be found.
Should the indentation be two spaces here?

Also might be worth adding a note in the source mirroring the info in the commit log so it's clearly documented why we're doing this.