Desired behavior when the reasoner cannot find a configuration/mode that fulfills current objective

[chcorbato]

(from #111 (comment))

@fmrico @norro @jginesclavero @ralph-lange in your pilots, what should be the behavior when the reasoner cannot find a configuration/mode that fulfils current objective?

Should the developer be able to define a DEFAULT FG/configuration/mode to choose when an objective is UNREACHABLE? or should it just report that it cannot be achieved and let the processing for the metacontroller/task planner?

this could also affect the @darkobozhinoski grammar/metamodel