Raise Error if no VOEvents to grab info from

We can get seemingly non-sequetar errors when a circular is being created for a superevent without any superevents: https://ligo-caltech.sentry.io/issues/3861647856/?project=1425216&referrer=issue-stream&statsPeriod=14d&stream_index=13

Instead we should raise an error if there are no VOEvents to draw info from.