Raise exception when one plan contradicts another

[clayote]

Currently there is no convenient way for the user to tell if a plan they are making contradicts one made previously. Contradicted plans are always canceled unceremoniously. If you want to handle contradiction some other way, like by making a new branch, you have to look at the future yourself and do it manually. It should be more convenient than that.

I'm thinking that when you contradict a plan outside of any planning block, then the plan should be canceled, but if you do so within a new planning block, an exception is raised instead.