add-apt-repository incorrect error msg for ipv6

[Matthias Arnason]

If you're unable to reach either of 91.189.89.22[23], add-apt-repository provides an error message stating that the user you provide does not exist, regardless of whether or not it does.

While this may be rare for most users, IPv6 only networks will always receive this misleading error message. Please consider detecting a failure to connect and distinguishing it from successfully connecting but receiving a negative response in the program's output.

[Launchpad Janitor (janitor)]

Status changed to 'Confirmed' because the bug affects multiple users.

[mpb (mpb)]

My Ubuntu 16.04 system does not have a default gateway.  Instead, I have configured apt-get to use an HTTP proxy.

Today I tried to add a ppa.

$ sudo add-apt-repository ppa:libreoffice/libreoffice-prereleases
Cannot add PPA: 'ppa:~libreoffice/ubuntu/libreoffice-prereleases'.
ERROR: '~libreoffice' user or team does not exist.

The error message is incorrect.  ~libreoffice does exist.

A better error message would be something like:

ERROR: unable to connect to remote host <hostname>

To reproduce the problem, simply run "sudo route del default", and then try to add a new ppa with add-apt-repository.

Bugs #1053531, #1433761 may be tangentially related.