Incremental upgrades: test if the USB stick is read-only

When accepting to upgrade with a read-only USB device, the error message is generic and points to .xsession-errors which doesn’t give a clue. That should be improved.

Error while installing the upgrade

An error occured while installing the upgrade.

Your Tails device needs to be repaired and might be unable to restart.

Please follow the instructions at file:///usr/share/doc/tails/website/doc/upgrade/error/install.en.html

For debugging information, see /home/amnesia/.xsession-errors

$ tail .xsession-errors

localuser:tails-upgrade-frontend being added to access control list
glibtop: Non-standard uts for running kernel:
release 3.12-1-amd64=3.12.0 gives version code 199680

The system is up-to-date
localuser:tails-upgrade-frontend being removed from access control list
** Message: NM disappeared

Parent Task: #7019

Original created by @alant on 7131 (Redmine)