Implement Docker Registry V2 error JSON

[hjacobs]

The stupid Docker client will not honor any standard HTTP status code (e.g. 401/403 will just result in "< nil >" printed by the Docker client) --- we need to implement the proprietary Docker error response objects (https://docs.docker.com/registry/spec/api/#errors-2) to get "proper" error messages on the console.

Example of "< nil >" when pushing without a valid OAuth token:

docker push pierone.example.org/myteam/myapp:0.2.1
The push refers to a repository [pierone.example.org/myteam/myapp] (len: 1)
68b46070c8ad: Preparing 
<nil>

[whiskeysierra]

👍

[tmuehl]

👍

[hjacobs]

See also moby/moby#18569