Comment 33 for bug 1371783

After some further investigating, I have some more information.

-- Comment #14 does indeed complete the installation but does not silence the popup. The similar method in Comment #26 does silence the popup.
-- It is right to be concerned that the font files could be corrupted, because they are hosted on SourceForge, which recently decided to wrap sleazeware around others' software (even though this is specifically prohibited in the fonts' EULA). See http://www.infoworld.com/article/2929732/open-source-software/sourceforge-commits-reputational-suicide.html
-- It might be best for someone else (GitHub? Ubuntu?) to host these files to make sure SourceForge doesn't corrupt them. It is possible that this (or the SourceForge "cookies" popup) is the root cause of this problem, though I have not pursued that. Also, I am often routed to the NetAssist mirror, which appears to have been taken down (it's online but empty).
-- In any case, the Ubuntu installer does verify the SHA256 hashes AFTER the download.

So this method should work (riffing off comments #14 and #26, along with a solution offered in http://askubuntu.com/questions/317765/how-do-i-stop-failure-to-download-extra-data-files-error-with-pepflashpluginin) ...

cd /var/lib/update-notifier/package-data-downloads/partial
grep Url: /usr/share/package-data-downloads/ttf-mscorefonts-installer | awk '{print $2}' | sudo xargs -n 1 'wget'
cd -
sudo /usr/lib/msttcorefonts/update-ms-fonts /var/lib/update-notifier/package-data-downloads/partial/*
sudo rm -f /var/lib/update-notifier/package-data-downloads/partial/*
sudo mv /var/lib/update-notifier/package-data-downloads/ttf-mscorefonts-installer.FAILED /var/lib/update-notifier/package-data-downloads/ttf-mscorefonts-installer