Document error codes

[nfroidure]

Find a way to document error codes by parsing the backend code and amending the Swagger on the fly.

[nfroidure]

Something like https://nodejs.org/en/blog/release/v8.0.0/#static-error-codes https://nodejs.org/dist/latest-v8.x/docs/api/errors.html#ERR_ARG_NOT_ITERABLE would be a perfect fit!