Detect that backend job failed

[aaannz]

When there happens some problem with main job, our users will know immediately as iguana will continue and fail with some message.
However when backend job fails, it is hidden in iguana and only visible in indirect ways, like when d-installer complains on dbus error.

Iguana should detect when backend job fails.

• print information about failed job to the screen
• enhance workflow to allow specifying if job is supposed to be restarted or not on failure