Error executing command, exiting -- Ошибка при обновлении/установки. E: Failed to fetch как исправить

Проблема

Если вы получаете список предупреждений вида:

W: The repository 'http://download.virtualbox.org/virtualbo... 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.
W: An error occurred during the signature verification. The repository is not updated and the previous index files will be used. GPG error: http://repo.mongodb.org/apt/ubuntu xenial/mongodb-org/3.2 Release: The following signatures were invalid: KEYEXPIRED 1507497109
W: The repository 'http://ppa.launchpad.net/rabbitvcs/ppa/u... 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: Failed to fetch http://repo.mongodb.org/apt/ubuntu/dists... The following signatures were invalid: KEYEXPIRED 1507497109
E: Failed to fetch http://ppa.launchpad.net/rabbitvcs/ppa/u... 404 Not Found
W: Some index files failed to download. They have been ignored, or old ones used instead.
W: Target Packages (partner/binary-amd64/Packages) is configured multiple times in /etc/apt/sources.list:43 and /etc/apt/sources.list.d/xenial-partner.list:4
W: Target Packages (partner/binary-i386/Packages) is configured multiple times in /etc/apt/sources.list:43 and /etc/apt/sources.list.d/xenial-partner.list:4
W: Target Packages (partner/binary-all/Packages) is configured multiple times in /etc/apt/sources.list:43 and /etc/apt/sources.list.d/xenial-partner.list:4
W: Target Translations (partner/i18n/Translation-en_US) is configured multiple times in /etc/apt/sources.list:43 and /etc/apt/sources.list.d/xenial-partner.list:4
W: Target Translations (partner/i18n/Translation-en) is configured multiple times in /etc/apt/sources.list:43 and /etc/apt/sources.list.d/xenial-partner.list:4
W: Target DEP-11 (partner/dep11/Components-amd64.yml) is configured multiple times in /etc/apt/sources.list:43 and /etc/apt/sources.list.d/xenial-partner.list:4
W: Target DEP-11-icons (partner/dep11/icons-64x64.tar) is configured multiple times in /etc/apt/sources.list:43 and /etc/apt/sources.list.d/xenial-partner.list:4
Error executing command, exiting

Решение

-- то необходимо исправить хотя самые критические, а именно ошибки ("E:")? в моём случае, это E: Failed to fetch....а именно:

E: Failed to fetch http://ppa.launchpad.net/rabbitvcs/ppa/u... 404 Not Found

Поэтому я поправил файл просто закоментировав в нем репозиторий:

 sudo nano /etc/apt/sources.list.d/rabbitvcs-ubuntu-ppa-xenial.list

(в общем случае ищите проблемный репозиторий в этих местах)