Use dlerror() to get error message when calling dlopen failed.

[minggangw]

Currently we will not check the return value of dlopen(), so when the dlopen failed, a nullptr will be passed to the rcl library, please see:

 const rosidl_message_type_support_t* ts =
      GetMessageTypeSupport(package_name, message_sub_folder, message_name);

  THROW_ERROR_IF_NOT_EQUAL(
      RCL_RET_OK,
      rcl_subscription_init(subscription, node, ts, topic.c_str(),
                            &subscription_ops),
rcl_get_error_string_safe());

We'd better check the |ts| and use dlerror() to get the error message to be returned.