在阿里雲安裝Ubuntu可視化桌面時，出現問題：Failed to fetch http://mirrors.cloud.aliyuncs.com/ubuntu/pool/main/x/xorg-server/xserver-xorg-core_1.18.4-0ubuntu0.3_amd64.deb 404 Not Found
https://www.cnblogs.com/jianglinliu/p/9999406.html
在阿里雲安裝Ubuntu可視化桌面時，出現問題：Failed to fetch http://mirrors.cloud.aliyuncs.com/ubuntu/pool/main/x/xorg-server/xserver-xorg-core_1.18.4-0ubuntu0.3_amd64.deb 404 Not Found
解決方法是：先用sudo apt-get update命令更新系統，然后用安裝命令
- apt-get install x-window-system-core
- apt-get install gnome-core
- apt-get install gdm
進行安裝即可