Document all the X-Error-Code in OpenAPI
and also in the docs, maybe, ie: a page with all the error types.

related to #1670 (I think we first want to generate the OpenAPI spec automatically, before documenting all the error codes)


Document all the X-Error-Code in OpenAPI: and also in the docs, maybe, ie: a page with all the error types.

related to #1670 (I think we first want to generate the OpenAPI spec automatically, before documenting all the error codes)

