backup: rework error reporting for subcommand

internal/archiver/file_saver.go

@@ -2,6 +2,7 @@ package archiver
 
 import (
 	"context"
+	"fmt"
 	"io"
 	"os"
 	"sync"
@@ -146,7 +147,7 @@ func (s *FileSaver) saveFile(ctx context.Context, chnker *chunker.Chunker, snPat
 				panic("completed twice")
 			}
 			isCompleted = true
-			fnr.err = err
+			fnr.err = fmt.Errorf("failed to save %v: %w", target, err)
 			fnr.node = nil
 			fnr.stats = ItemStats{}
 			finish(fnr)

internal/fs/fs_reader_command.go

@@ -83,7 +83,7 @@ func (fp *CommandReader) wait() error {
 	err := fp.cmd.Wait()
 	if err != nil {
 		// Use a fatal error to abort the snapshot.
-		return errors.Fatal(err.Error())
+		return errors.Fatal(fmt.Errorf("command failed: %w", err).Error())
 	}
 	return nil
 }