Update for ErrUnauthorizedForCredentials API change in containers/image
Update for ErrUnauthorizedForCredentials API change in containers/image

Replace an equality check with a type assertion when checking for a
docker.ErrUnauthorizedForCredentials in `podman login`.

Signed-off-by: Nalin Dahyabhai <nalin@redhat.com>

Closes: #1902
Approved by: rhatdan

TomSweeneyRedHat/buildah,nalind/buildah-1,projectatomic/buildah,nalind/buildah,TomSweeneyRedHat/buildah,nalind/buildah,projectatomic/buildah,rhatdan/buildah,rhatdan/buildah,rhatdan/buildah,nalind/buildah-1,projectatomic/buildah,TomSweeneyRedHat/buildah,nalind/buildah-1