List a missing file:// destination directory as an error (closes #220)

The file backend listed a destination directory that does not exist as
an empty store. With the volume unplugged, snapshot list reported every
local snapshot as missing from the store, snapshot remove said it had
removed metadata it never reached, and prune dropped every local
snapshot record. List and ListStream now fail when the destination
directory is missing, so those commands take their existing path for a
store that cannot be listed. A missing prefix under an existing
directory is still an empty listing, and a first backup still creates
the directory.

Three tests listed a file:// destination nothing had created; they now
create it.

Model: opus-5-5

internal/storage/file_backend_test.go

@@ -1,6 +1,10 @@
 package storage_test
 
 import (
+	"context"
+	"errors"
+	"io/fs"
+	"path/filepath"
 	"testing"
 
 	"sneak.berlin/go/vaultik/internal/storage"
@@ -25,3 +29,34 @@ func TestFileStorer(t *testing.T) {
 	t.Parallel()
 	runStorerConformance(t, newFileStorer)
 }
+
+// TestFileStorerListMissingDestination checks that List and ListStream
+// fail when the destination directory does not exist, instead of
+// reporting an empty store.
+func TestFileStorerListMissingDestination(t *testing.T) {
+	t.Parallel()
+
+	ctx := context.Background()
+
+	s, err := storage.NewFileStorer(filepath.Join(t.TempDir(), "unmounted"))
+	if err != nil {
+		t.Fatalf("NewFileStorer: %v", err)
+	}
+
+	keys, err := s.List(ctx, "metadata/")
+	if !errors.Is(err, fs.ErrNotExist) {
+		t.Errorf("List = %v, %v; want a not-exist error", keys, err)
+	}
+
+	var streamErr error
+
+	for object := range s.ListStream(ctx, "metadata/") {
+		if object.Err != nil {
+			streamErr = object.Err
+		}
+	}
+
+	if !errors.Is(streamErr, fs.ErrNotExist) {
+		t.Errorf("ListStream error = %v, want a not-exist error", streamErr)
+	}
+}