ATM, if there is problem with input to discount-code-importer it throws an generic exception and doesn't tell us what is wrong and why it is failed.
This package is being used by the express-impex and hence the customers are not able to understand about the error and reach out to customer support to think its an bug.
Generic Error
Currently user sees this exception incase of any error however specialy it should also show actual error like
error: Cart Discount '0bfd0919-0540-4350-a6af-16ffa913a954' doesn't require a discount code.
Support reference number
DiscountCodeImporter shows generic exception
Generic Error
error: Cart Discount '0bfd0919-0540-4350-a6af-16ffa913a954' doesn't require a discount code.