fix(install): stop when a server has no download configuration (#4996)

fn_install_server_files logged the failure but carried on, then tried to
fetch and extract with an empty URL. Exit after the failure instead.

Found in #4155 by @jusito.

lgsm/modules/install_server_files.sh

@@ -220,6 +220,7 @@ fn_install_server_files() {
 	else
 		fn_print_fail_nl "Installing ${gamename} Server failed, missing default configuration"
 		fn_script_log_fail "Installing ${gamename} Server failed, missing default configuration"
+		core_exit.sh
 	fi
 	fn_fetch_file "${remote_fileurl}" "" "" "" "${local_filedir}" "${local_filename}" "${chmodx}" "${run}" "${forcedl}" "${md5}"
 	fn_dl_extract "${local_filedir}" "${local_filename}" "${serverfiles}"