That error would happen when the gpg key used to sign the packages is no longer on the keyserver run by launchpad. That is not the case, I have tested just now and did not reproduce that problem in my environment.
It is possible that your system is unable to connect to
keyserver.ubuntu.com, perhaps because of a proxy in the way, or other networking issue. The apt-get update command might have also failed, however, in that case.
The other possibility is that
kerserver.ubuntu.com was down or unavailable at the time you tried. That should not happen, but it is conceivable.