Deal with exceptions

[Ricardo Maia Avelino]

I noticed that when there's an error in the calculation, as for example, no independent are selected (no assigned forces), or even if the modification in the force diagram result in an error of this sort. How should we overcome this? Perhaps if the functions form_from_qid and force_from_form throw an exception this would do? and we can show the user a proper message?

[Tom Van Mele]

this is now indirectly solved...