Without proofing the actual execution, it’s not unreasonable to do this.
You may also want to look at add_error as well as traverse_errors.
Most of my code has separate schemas for the data layer (business logic focused) and UI layer (user interface focused, may combine or obscure mulitiple data layer schemas - ex: data layer price is always cents but the ui layer changeset may accept dollars + cents), so with that in mind I would probably attach the error-code stuff to that as just another validation step. I may Enum.reduce(changeset.errors, changeset, ...) and just insert the error code into the existing errors.
I also would tend to return {:error, changeset} only because of convention, vs {:error, errors} but this is really code base specific – just be consistent.
