Avoid swallowing error output from apt_get_install

[johnsca]

A user was getting intermittent install hook failures, with the relevant logs here: http://pastebin.ubuntu.com/13503590/

This seems like it could be caused by network issues on the user's environment, but the error from apt-get is getting swallowed by the charm. This should allow the error to show up in the log.

[hatched]

This looks great, thanks for the PR @johnsca :+1:

[urosj]

:+1: Thanks for your commit, @johnsca

[urosj]

:shipit: