在阿里云安装Ubuntu可视化桌面时，出现问题：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
解决方法是：先用sudo apt-get update命令更新系统，然后用安装命令
- apt-get install x-window-system-core
- apt-get install gnome-core
- apt-get install gdm
进行安装即可