docker: reuse docker/distribution registry errros

[runcom]

Instead of that big json, reuse docker/distribution registry errors since we are already used by that in Docker. This change isn't a big deal but at least I got tests working in projectatomic/docker#200 and errors are much much nicer with skopeo:

$ ./skopeo inspect docker://runcom/doesnotexist
FATA[0001] manifest unknown: manifest unknown

$ ./skopeo inspect docker://asdasd
FATA[0002] unauthorized: authentication required

Note those errors above are the same docker print out.

This is a blocker for projectatomic/docker#200 because w/o this we don't have any way/mapping to tell docker to fallback to v1 and we end up breaking docker.

@mtrmac PTAL

@baude @aweiteka sort of related to projectatomic/atomic#636

Signed-off-by: Antonio Murdaca [email protected]

[runcom]

@glestaris PTAL since you exported that ErrFetchManifest but you can now use finer grained errors from docker/distribution

[runcom]

skopeo's PR to fix tests containers/skopeo#232

[mtrmac]

👍

[glestaris]

@runcom thanks for the heads up! We got it bumpbed yesterday cloudfoundry/grootfs@0d2ed86 :)