Restore the other files when a chunk is in no blob under --skip-errors (closes #326)

Restore planning stopped at the first file needing a chunk that no blob
holds, so --skip-errors never saw that file and nothing was restored.
The planner now leaves such a file out of the plan and returns it with
the reason, and each one goes through handleRestoreFileError like any
other failed file: skipped and counted under --skip-errors, fatal
without it. The fatal error now suggests --skip-errors, as other restore
failures do.

The skip-errors tests now share one backup helper; the new case removes
a blob from the snapshot metadata and exports it again.

Model: opus-5-5

internal/vaultik/restore.go

@@ -309,11 +309,22 @@ func (v *Vaultik) restoreAllFiles(
 		return nil, err
 	}
 
-	plan, err := newRestorePlan(v.ctx, repos, files, chunkToBlobMap, blobIDToHash)
+	plan, unplanned, err := newRestorePlan(
+		v.ctx, repos, files, chunkToBlobMap, blobIDToHash)
 	if err != nil {
 		return nil, fmt.Errorf("building restore plan: %w", err)
 	}
 
+	// Ranging over files rather than the map reports them in a stable order.
+	for _, file := range files {
+		if planErr, ok := unplanned[file.ID]; ok {
+			err = v.handleRestoreFileError(plan, opts, result, file, file.ID, planErr)
+			if err != nil {
+				return nil, err
+			}
+		}
+	}
+
 	filesByID, totalBytesExpected := indexRestoreFiles(files)
 
 	v.UI.Beginf("Restoring %s files (%s) to %s.",