Improve Tails Installer robustness for 2.6

This ticket is about the iteration aimed at 2.6 of the improvements I’ve been working on as part of #10720 (closed). The goal is to get the installer robustness & debugging improvements produced there into 2.6, so that they benefit human users even though they might not be good enough to mark the tests as non-fragile on Jenkins yet (and close #10720 (closed)).

Feature Branch: bugfix/11590-installer-robustness, installer:bugfix/10720-installer-freezes-on-jenkins

Parent Task: #10988 (closed)

Related issues

Blocks #11582 (closed)
Blocks #11588 (closed)

Original created by @intrigeri on 11590 (Redmine)

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