As an admin importing data with relationship columns, I want the import to tell me if a related object doesn't exist

[kitsuta]

Right now, although we explicitly check to see if the id column exists, we do no validations on IDs that link to other tables.

For example, if you're trying to import games owned by people, and you fill the attendee_id column with IDs, any ID that doesn't exist in the DB will cause the unhelpful "Import unsuccessful" error because it fails at the session.commit() step. Ideally, what would instead happen is that it imports all the rows with working attendee_id columns, and prints a list of the failed rows.