Docker unauthenticated access shows "unknown: unknown" console output

Description

When having configured a docker proxy without or with invalid HTTP Authentication we expect a similar or at least close to output message as docker hub would give us.

To reproduce

1. Setup a private repo on Docker Hub (see here for some instructions on a setup)
2. Setup a docker proxy, do not give it any HTTP Authentication
3. On the command do a pull
   1. docker pull <ip:port>/<username>/<reponame>

Actual

Using default tag: latest
Error response from daemon: unknown: unknown

Expected

Using default tag: latest
Error response from daemon: repository billybob/hello-test-1234 not found: does not exist or no pull access