Add some error context in Changes codepaths

Motivated by containers#2042
where we just get a bare `invalid argument` out of the entire
storage stack.

My offhand guess skimming some of the code is by far
the most likely thing here is the raw `lgetxattr` call. It'd
be useful to know that for sure.

Signed-off-by: Colin Walters <[email protected]>

drivers/overlay/overlay.go

@@ -2433,14 +2433,18 @@ func (d *Driver) Changes(id string, idMappings *idtools.IDMappings, parent strin
 	// layers.
 	diffPath, err := d.getDiffPath(id)
 	if err != nil {
-		return nil, err
+		return nil, fmt.Errorf("failed to get diff path: %w", err)
 	}
 	layers, err := d.getLowerDiffPaths(id)
 	if err != nil {
-		return nil, err
+		return nil, fmt.Errorf("failed to get lower diff path: %w", err)
 	}
 
-	return archive.OverlayChanges(layers, diffPath)
+	c, err := archive.OverlayChanges(layers, diffPath)
+	if err != nil {
+		return nil, fmt.Errorf("computing changes: %w", err)
+	}
+	return c, nil
 }
 
 // AdditionalImageStores returns additional image stores supported by the driver

pkg/archive/changes_linux.go

@@ -316,7 +316,11 @@ func parseDirent(buf []byte, names []nameIno) (consumed int, newnames []nameIno)
 // with respect to the parent layers
 func OverlayChanges(layers []string, rw string) ([]Change, error) {
 	dc := func(root, path string, fi os.FileInfo) (string, error) {
-		return overlayDeletedFile(layers, root, path, fi)
+		r, err := overlayDeletedFile(layers, root, path, fi)
+		if err != nil {
+			return "", fmt.Errorf("overlay deleted file query: %w", err)
+		}
+		return r, nil
 	}
 	return changes(layers, rw, dc, nil, overlayLowerContainsWhiteout)
 }
@@ -351,7 +355,7 @@ func overlayDeletedFile(layers []string, root, path string, fi os.FileInfo) (str
 	// If the directory isn't marked as opaque, then it's just a normal directory.
 	opaque, err := system.Lgetxattr(filepath.Join(root, path), getOverlayOpaqueXattrName())
 	if err != nil {
-		return "", err
+		return "", fmt.Errorf("failed querying overlay opaque xattr: %w", err)
 	}
 	if len(opaque) != 1 || opaque[0] != 'y' {
 		return "", err