Error trapping on discount codes based on Cart contents
So this is for coupons?

If a coupon can't be applied during an updateCart, you'll get back the rather unhelpful response in JSON of {"error":"Cart not completely updated."}

We intercept this, and because we know it's a coupon operation just replace the text of that to 'Could not apply coupon'.

But it does not give you a reason or anything, unfortunately.