Issues uploading when there's no election date

[GeoWill]

Issue and Steps to Reproduce

When uploading a file for a council with no election (go to /uploads/upload_files/<reg-code>/) the user sees 'something's gone wrong', while we just raise a 500, because we tried to create an Upload instance with no election date. Example.

The fix for this is probably to do something around where we're setting the election date, and not let the user even push the upload button - instead we should say 'we don't know about any upcoming elections' please tell us...'.

Alternatively we could not even let anything happen if UPCOMING_ELECTION_DATES is empty. Maybe this is better 🤔