reader reports tokenizer errors through normal return

src/reader/exprstatestack.cpp

@@ -20,6 +20,11 @@ namespace xo {
             return *(stack_[z-1]);
         }
 
+        void
+        exprstatestack::reset_to_toplevel() {
+            this->stack_.resize(1);
+        }
+
         void
         exprstatestack::push_exprstate(std::unique_ptr<exprstate> exs) {
             constexpr bool c_debug_flag = true;