prevent bogus manifest fetching success

An error while fetching an image manifest would return a nil
error (hence indicating success) with a unit value image.Info{}
struct.

That is bad news for the caller in Warmer.warm(), which will map an
image tag to that empty image.Info{}, polluting the cache entry for
the image+tag and image in memcached.

When we subsequently use this info to determine the latest suitable
tag, we encounter zero CreatedAt timestamps, which, prior to the
changes in #1247, #1249 and #1250 would cause the wrong images to be
released.

Fixes #1127.

registry/client.go

@@ -79,7 +79,7 @@ func (a *Remote) Manifest(ctx context.Context, ref string) (image.Info, error) {
 
 interpret:
 	if fetchErr != nil {
-		return image.Info{}, err
+		return image.Info{}, fetchErr
 	}
 
 	info := image.Info{ID: a.repo.ToRef(ref), Digest: manifestDigest.String()}