Error with proxied images: No 'Authorization' header provided in request

[divolgin]

What I Did

Fixed error handling and error messaging for proxied private images when those images do not exist in the private registry.

How I Did it

Added handling for "not found" error and removed the unauthenticated pull attempt.

How to verify it

Add a bogus image to assets (linked registry must be setup properly for this), such as

    - docker:
        image: myorg/myimage:latestest
        source: gcr
        dest: docker/myimage:latestest.tar

Error should read

Error response from daemon: manifest for myimage:latestest not found

Instead of

unauthorized: No 'Authorization' header provided in request

Description for the Changelog

Better user messaging when specified images do not exist in the private registry.

Picture of a Ship (not required but encouraged)

[dexhorthy]

lgtm, one suggestion