tl;dr:A valuable error message should: (1) Use the correct HTTP status code. (2) Wrap the error in an “error” object. (3) Be helpful by providing the error code. (4) The error type. (5) A link to the relevant docs. (6) The API version used in this request. (7) A suggestion on how to fix the issue. Paul shares an example by Stripe.
tl;dr:“You might disagree with how the Stripe API is designed, and the design you end up with is likely going to be different than what we use. That’s just fine, since different companies have different use cases. Instead I present here some design patterns that I believe are generic enough to be useful for just about anyone in the API design process.”