Improve error message when instantiating virtual class

(cherry picked from commit 29dc8de88bdba441b64197712d52c43bbdf6ee1c)

core/class_db.cpp

@@ -509,7 +509,7 @@ Object *ClassDB::instance(const StringName &p_class) {
 		}
 		ERR_FAIL_COND_V_MSG(!ti, nullptr, "Cannot get class '" + String(p_class) + "'.");
 		ERR_FAIL_COND_V_MSG(ti->disabled, nullptr, "Class '" + String(p_class) + "' is disabled.");
-		ERR_FAIL_COND_V(!ti->creation_func, nullptr);
+		ERR_FAIL_COND_V_MSG(!ti->creation_func, nullptr, "Class '" + String(p_class) + "' or its base class cannot be instantiated.");
 	}
 #ifdef TOOLS_ENABLED
 	if (ti->api == API_EDITOR && !Engine::get_singleton()->is_editor_hint()) {