core/oci: Ignore NotAnImage errors when deleting image

If image doesn't exist, containers returns ErrNotAnImage, which is
expected when the image doesn't exist. This is the case when first
updating the system, for example.

core/oci.go

@@ -20,6 +20,7 @@ import (
 	"path/filepath"
 
 	"github.com/containers/buildah"
+	cstypes "github.com/containers/storage/types"
 	"github.com/vanilla-os/abroot/settings"
 	"github.com/vanilla-os/prometheus"
 )
@@ -178,7 +179,7 @@ func DeleteImageForRoot(root string) error {
 	}
 
 	_, err = pt.Store.DeleteImage(image, true)
-	if err != nil {
+	if err != nil && err != cstypes.ErrNotAnImage {
 		PrintVerbose("DeleteImageForRoot:err(3): %s", err)
 		return err
 	}