Allow rethrowing `MRB_TT_BREAK`

src/error.c

@@ -241,10 +241,15 @@ mrb_exc_set(mrb_state *mrb, mrb_value exc)
 MRB_API mrb_noreturn void
 mrb_exc_raise(mrb_state *mrb, mrb_value exc)
 {
-  if (!mrb_obj_is_kind_of(mrb, exc, mrb->eException_class)) {
-    mrb_raise(mrb, E_TYPE_ERROR, "exception object expected");
+  if (mrb_break_p(exc)) {
+    mrb->exc = mrb_obj_ptr(exc);
+  }
+  else {
+    if (!mrb_obj_is_kind_of(mrb, exc, mrb->eException_class)) {
+      mrb_raise(mrb, E_TYPE_ERROR, "exception object expected");
+    }
+    mrb_exc_set(mrb, exc);
   }
-  mrb_exc_set(mrb, exc);
   if (!mrb->jmp) {
     mrb_p(mrb, exc);
     abort();