Overhaul of thread handling to support blocking functions other than wait

luprex/core/cpp/traceback.cpp

@@ -76,12 +76,19 @@ int traceback_coroutine(lua_State *L) {
 }
 
 
-int traceback_pcall(lua_State *L, int narg, int nret) {
+eng::string traceback_pcall(lua_State *L, int narg, int nret) {
     int status;
     int base = lua_gettop(L) - narg; /* function index */
     lua_pushcfunction(L, traceback_coroutine); /* push traceback function */
     lua_insert(L, base);             /* put it under chunk and args */
     status = lua_pcall(L, narg, nret, base);
     lua_remove(L, base); /* remove traceback function */
-    return status;
+    if (status != LUA_OK) {
+        const char *msg = lua_tostring(L, -1);
+        if ((msg == NULL) || (msg[0] == 0)) {
+            msg = "unknown error";
+        }
+        return msg;
+    }
+    return "";
 }