Fix orphaned orders and duplicate transaction reference errors

🐛 We had a bug where if a merchant submitted a duplicate transaction reference, they were presented with a 500 error and an orphaned order row was created in the DB. This has now been resolved and the merchant received a validation error on the merchant_transaction_reference field instead. No orphaned order row is created. This also resolved a problem where a lot of accounts couldn't run data exports.

🧑‍💻 Jules