Tails Installer's is_ISO_selected method is not a predicate

From #8860-note_40:

The is_ISO_selected name suggests it’s a simple predicate but this method 1. checks if we need an ISO in the first place (which the name doesn’t convey); 2. has side effects (while a predicate must not). I think some refactoring is in order. From a caller’s PoV I would do:

if self.need_ISO() and not self.is_ISO_selected():
    self.warn_ISO_not_selected()

… i.e. two predicates and a third method that does the side-effect.

Feature Branch: tails:bugfix/14755-installer-consistent-drive-plugging-state

Parent Task: #9005 (closed)

Original created by @anonym on 14722 (Redmine)