我通过参考zcash 安装步骤在服务器（Ubuntu）上安装 Zcash
但是当我在终端中输入以下命令时，出现错误
sudo apt-get update Ign:1 http://deb.debian.org/debian stretch InRelease Hit:2 http://security.debian.org stretch/updates InRelease Hit:3 http://deb.debian.org/debian stretch-updates InRelease Get:4 http://repo.stackdriver.com/apt stretch InRelease [2,100 B] Hit:5 http://deb.debian.org/debian stretch-backports InRelease Hit:6 http://deb.debian.org/debian stretch Release Hit:7 http://packages.cloud.google.com/apt google-cloud-logging-wheezy InRelease Hit:8 http://packages.cloud.google.com/apt google-cloud-monitoring-stretch InRelease Hit:9 http://packages.cloud.google.com/apt cloud-sdk-stretch InRelease Hit:10 http://packages.cloud.google.com/apt google-compute-engine-stretch-stable InRelease ............ Err:19 http://ppa.launchpad.net/ubuntu-toolchain-r/test/ubuntu cosmic/main amd64 Packages 404 Not Found Ign:20 http://ppa.launchpad.net/ubuntu-toolchain-r/test/ubuntu cosmic/main Translation-en Ign:21 http://ppa.launchpad.net/ubuntu-toolchain-r/test/ubuntu cosmic/main Translation-en_US Ign:22 http://ppa.launchpad.net/ubuntu-toolchain-r/test/ubuntu cosmic/main Translation-en_US.UTF-8 Fetched 2,100 B in 6s (323 B/s) Reading package lists... Done W: GPG error: http://repo.stackdriver.com/apt stretch InRelease: The following signatures were invalid:B10FDCDCEC088467D0069F423C6E15887B190BD2 W: The repository 'http://repo.stackdriver.com/apt stretch 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. W: The repository 'http://ppa.launchpad.net/ubuntu-toolchain-r/test/ubuntu cosmic 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.
什么GPG错误是什么意思？如何验证 GPG 签名？