Improve context for errors in PrepareStagedLayer

- Diferentiate partial pull errors from other read errors
- Indicate that this is PrepareStagedLayer that is failing

Signed-off-by: Miloslav Trmač <mitr@redhat.com>

copy/single.go

@@ -828,7 +828,7 @@ func (ic *imageCopier) copyLayer(ctx context.Context, srcInfo types.BlobInfo, to
 			return false, types.BlobInfo{}, err
 		}()
 		if err != nil {
-			return types.BlobInfo{}, "", fmt.Errorf("reading blob %s: %w", srcInfo.Digest, err)
+			return types.BlobInfo{}, "", fmt.Errorf("partial pull of blob %s: %w", srcInfo.Digest, err)
 		}
 		if reused {
 			return blobInfo, cachedDiffID, nil

storage/storage_dest.go

@@ -335,7 +335,7 @@ func (s *storageImageDestination) PutBlobPartial(ctx context.Context, chunkAcces
 
 	out, err := s.imageRef.transport.store.PrepareStagedLayer(nil, differ)
 	if err != nil {
-		return private.UploadedBlob{}, err
+		return private.UploadedBlob{}, fmt.Errorf("staging a partially-pulled layer: %w", err)
 	}
 	succeeded := false
 	defer func() {