Instruct people with a version older than n-2 to do manual upgrade in /doc/upgrade/error/check

This page is displayed locally by Tails Upgrader if checking for upgrades fails. See #12392-note_3.

Original created by @sajolida on 12492 (Redmine)

Edited Nov 22, 2019 by intrigeri

To upload designs, you'll need to enable LFS and have an admin enable hashed storage. More information