PPA 似乎一直处于离线状态。每当我使用时sudo apt-get update，都会显示此错误：
W: Failed to fetch http://ppa.launchpad.net/ 404 Not Found
如何修复这些错误？
每次我尝试执行软件更新时，我都会从软件管理器收到此错误：
W:GPG error: http://download.virtualbox.org/virtualbox/debian xenial InRelease: The following signatures couldn't be verified because the public key is not available: NO_PUBKEY A2F683C52980AECF, W:The repository 'http://download.virtualbox.org/virtualbox/debian xenial InRelease' is not signed., W:Data from such a repository can't be authenticated and is therefore potentially dangerous to use., W:See apt-secure(8) manpage for repository creation and user configuration details., W:There is no public key available for the following key IDs:
A2F683C52980AECF , W:The repository 'http://ppa.launchpad.net/blackmage/f.lux/ubuntu xenial Release' does not have a Release file., W:Data from …