Please add a webhook for failed transactions, no matter the reason (expired, processing error, general error, whatever).
I'm not sure what kind of company wouldn't want to know immediately and automatically about a failed transaction, especially if it's for a subscription (ARB).
Silent Post does this and we're trying to migrate away from it per customer support advice but glaring omissions like this are making it near impossible.
The only solution is to either constantly query the API for unsettled txns to find failures (if we want to know about them immediately, which we do), or if we didn't mind waiting up to 24 hours then we could query once per day for the entire batch for the previous day to get all transactions and find the failed ones - but this is 2018 damnit, everything should be real time.