Comment 25 for bug 1543280

encountered this today. Resulted in an "information available" window opening that appears forever unsatisfied ("run this action now" button doesn't make it go away, even after applying the fix at bottom)
Information available dialogue is as follows:
Failure to download extra data files
The following packages requested additional data downloads after package installation, but the data could not be downloaded or could not be processed.
ttf-mscorefonts-installer
The download will be attempted again later, or you can try the download again now. Running this command requires an active Internet connection.

The following steps succeeded and installed the troublesome ttf-mscorefonts-installer:

$ sudo chown _apt:root /var/lib/update-notifier/package-data-downloads/partial/

$ sudo rm /var/lib/update-notifier/package-data-downloads/partial/*.FAILED

$ sudo apt install --reinstall update-notifier-common

runs to completion, final 2 lines =

All done, no errors.
All fonts downloaded and installed.