The key point above is that:
sudo apt-add-repository ppa:juju/pkgs --yes
failed to import keys to apt, but
a.) exited success (0) meaning its caller did not even log error
b.) left /etc/apt/sources.list.d/<ppa>.list in place, meaning subsequent 'apt-get update' was guaranteed to fail.
The key point above is that: sources. list.d/ <ppa>.list in place, meaning subsequent 'apt-get update' was guaranteed to fail.
sudo apt-add-repository ppa:juju/pkgs --yes
failed to import keys to apt, but
a.) exited success (0) meaning its caller did not even log error
b.) left /etc/apt/