Step "I update APT using apt-get" succeeds even if apt-get update failed

Apparently, on apt-get failure we display STDERR but don’t fail the step:

  Scenario: Install packages using apt-get                               # features/apt.feature:25

We trust you have received the usual lecture from the local System
Administrator. It usually boils down to these three things:

    #1) Respect the privacy of others.
    #2) Think before you type.
    #3) With great power comes great responsibility.

[sudo] password for amnesia: W: Failed to fetch http://ftp.us.debian.org/debian/dists/experimental/main/i18n/Translation-en  Hash Sum mismatch

E: Some index files failed to download. They have been ignored, or old ones used instead.
    When I update APT using apt-get                                      # features/step_definitions/apt.rb:15

… and the test passes.

Feature Branch: kytv:test/8697-update-apt-get

Parent Task: #8539 (closed)

Original created by @intrigeri on 8697 (Redmine)