backend/repository: assume no v2 on unexpected status
backend/repository: assume no v2 on unexpected status

This commit changes version detection behavior to align with
Registry v2 specs:

```
If 404 Not Found response status, or other unexpected status,
is returned, the client should proceed with the assumption
that the registry does not implement V2 of the API.
```

Reference: https://docs.docker.com/registry/spec/api/#/api-version-check

dgonyeo/docker2aci,appc/docker2aci,appc/docker2aci,OMWAVE/docker2aci,OMWAVE/docker2aci,dgonyeo/docker2aci