fix: Additional checks for closing orders/settling markets

[eoin-betdex]

Tightening up operations to settle/close some of the accounts.

1. Prevent market settlement completing while there is still escrow funds to be returned, i.e., there must still be unsettled orders. This check will mean that by the time close_market is called there can be non-settled orders. At least all winnings should have been paid out.
2. Throw an error if an order that is attempted to be closed is not yet complete, i.e., settled or cancelled.

Ideally close_market could be restricted while there still are dependent accounts open but I don't thinkthis is possible with the current data model. E.g., be sure all cancelled orders are closed first.