Fix crash in filesystem API.

api/fs.go

@@ -42,6 +42,9 @@ func (c *FSClient) Mkdir(ctx context.Context, paths ...string) error {
 
 func (c *FSClient) ReadDir(ctx context.Context, dir string) ([]FileInfo, error) {
 	res, err := c.client.ReadDir(ctx, &rpc.PathRequest{Path: dir})
+	if err != nil {
+		return nil, err
+	}
 	return convertEntries(res.Entries), err
 }