Report more precisely why an incremental upgrade cannot be done

See discussion around 2013-12-26 and 2014-01-07, in the “Please review draft documentation for IUK” thread on tails-dev.

Feature Branch: feature/6508-incremental-upgrades-phase-four

Original created by @intrigeri on 6575 (Redmine)

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