[fud2] Allow planners to return errors

[jku20]

Currently planners can only communicate their failure to find a plan by returning None. As planners become more complicated, it is possible for them to fail in other ways too, for example, JsonPlanner reads from json files which can be potentially malformed. Having find_plan return a Result instead of an Option could be a good solution here.

This requires thinking hard about what fud2 errors should look like, which may or may not be hard.