fix: align WebDAV MOVE overwrite and source errors

chfs-ng/internal/api/server.go

@@ -914,9 +914,11 @@ func (f *webdavStorageFS) Copy(ctx context.Context, name, dest string, opts *web
 func (f *webdavStorageFS) Move(ctx context.Context, name, dest string, opts *webdav.MoveOptions) (bool, error) {
 	src := davStoragePath(name)
 	dst := davStoragePath(dest)
-	if opts != nil && opts.NoOverwrite {
+	if opts == nil || opts.NoOverwrite {
 		if _, err := f.store.Stat(dst); err == nil {
-			return false, fmt.Errorf("destination exists")
+			return false, webdav.NewHTTPError(http.StatusPreconditionFailed, fmt.Errorf("destination exists"))
+		} else if !os.IsNotExist(err) {
+			return false, err
 		}
 	}
 	if m, ok := f.store.(storage.Multi); ok {
@@ -926,7 +928,16 @@ func (f *webdavStorageFS) Move(ctx context.Context, name, dest string, opts *web
 			return false, webdav.NewHTTPError(http.StatusForbidden, fmt.Errorf("cross-root move is not supported"))
 		}
 	}
+	if _, err := f.store.Stat(src); err != nil {
+		if os.IsNotExist(err) {
+			return false, webdav.NewHTTPError(http.StatusNotFound, err)
+		}
+		return false, err
+	}
 	if err := f.store.Rename(src, dst); err != nil {
+		if os.IsNotExist(err) {
+			return false, webdav.NewHTTPError(http.StatusNotFound, err)
+		}
 		return false, webdav.NewHTTPError(http.StatusForbidden, err)
 	}
 	return true, nil