Why can't apps modify the original HTTP error message, for example using "403 Not Enough Credit" instead of "403 Forbidden", in their first use case?
We are using this format for quite some time and it is very easy to use. In above example "Not Enough Credit" sounds reasonable, but you often want to inform users with more information: what was the transactional amount, what is the balance, are there other reasons the transaction failed? You can easily extend the response with more fields to indicate such thing.
Besides, you can track any error in your backoffice system and link the error number in your response. This allows you to have direct insights in error history for customer support ("Hey I got an error and the system said to report number 23473 to you, can you help me?").
Their example is the error 'You do not have enough credit'. Surely their web server should be sending '403 You do not have enough credit' and not '403 Forbidden' ?