linux之在 Ubuntu 16.04 上执行 sudo apt-get update 命令后出现以下错误

我对 Linux 命令一无所知，但我忍不住要问为什么在命令 sudo apt-get update 之后会出现这样的错误。 :

W: The repository 'http://ppa.launchpad.net/bumblebee/stable/ubuntu xenial Release' does not have a Release file.

N: Data from such a repository can't be authenticated and is therefore potentially dangerous to use.

N: See apt-secure(8) manpage for repository creation and user configuration details.

W: The repository 'http://ppa.launchpad.net/ubuntu-x-swat/x-updates/ubuntu xenial Release' does not have a Release file.

N: Data from such a repository can't be authenticated and is therefore potentially dangerous to use.

N: See apt-secure(8) manpage for repository creation and user configuration details.

W: GPG error: http://ppa.launchpad.net/umang/indicator-stickynotes/ubuntu xenial InRelease: The following signatures couldn't be verified because the public key is not available: NO_PUBKEY 799A5FD5D1C5997F

W: The repository 'http://ppa.launchpad.net/umang/indicator-stickynotes/ubuntu xenial InRelease' is not signed.

N: Data from such a repository can't be authenticated and is therefore potentially dangerous to use.

N: See apt-secure(8) manpage for repository creation and user configuration details.

E: Failed to fetch http://ppa.launchpad.net/bumblebee/stable/ubuntu/dists/xenial/main/binary-amd64/Packages 404 Not Found

E: Failed to fetch http://ppa.launchpad.net/ubuntu-x-swat/x-updates/ubuntu/dists/xenial/main/binary-amd64/Packages 404 Not Found

E: Some index files failed to download. They have been ignored, or old ones used instead.

请您参考如下方法:

我按照以下步骤解决了这个错误:

打开软件更新应用程序；
- 在“其他软件”选项卡中取消标记所有选项；
- 在 Ubuntu 软件选项卡中，“下载自:”选择“主服务器”选项。

单击“关闭”按钮并等待一分钟，直到 ubuntu 重新加载 apt-get。

完毕。