Send unavailable presence to all muc when the IRC server closes the connection

src/irc/irc_client.cpp

@@ -46,6 +46,8 @@ void IrcClient::on_connection_close()
 {
   static const std::string message = "Connection closed by remote server.";
   this->send_gateway_message(message);
+  const IrcMessage error{"ERROR", {message}};
+  this->on_error(error);
   log_warning(message);
 }