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

Originally created by @sajolida on #12492 (Redmine)

This page is displayed locally by Tails Upgrader if checking for upgrades fails. See #12392 (comment 47266).

Feature Branch: doc/12492-manual-upgrade-on-errors

Edited May 15, 2020 by sajolida

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