Use URI not codes for error messages

[ioggstream]

I suggest

to consider URIs (e.g https://immuni.example/errors/1004) instead of error-codes (eg. 1004) to describe errors. See https://tools.ietf.org/html/rfc7807#section-4.

The app can still process the response, thus deriving the error code.

This simplifies the evolution of the API interface and better guides the client in resolving possible errors conditions without filing issues or even contacting the support.