Improve output feedback and information while Maestro executes an action

Signed-off-by: Maxime Petazzoni <max@signalfuse.com>
Correctly implement reverse-order for stopping services

Code refactoring in dependency gathering to support both directions
(requires and depends on), to correctly implement reverse-order for
stopping services.

A few misc tweaks in display on errors too.

Signed-off-by: Maxime Petazzoni <max@signalfuse.com>