Re-evaluate (in 11/2017) error response formats

[chludwig-haufe]

As of now, the section on error handling references both the vnd.error draft and RFC 7807.

At some point (proposal: in 6 months from now, i.e. November 2017), we should re-evaluate whether it makes more sense to settle for one of the formats. One of our tenets is that familiar formats make it easier for developers to use an API - if we stick with one format for future APIs, then chances are better that developers know it already.