Raise MissingGameSystem exception

When there's no game system present, and we'r trying to calculate
`Character#level_up?`, we should raise a more descriptive exception than
`NoMethodError`.

This patch implements the `Character::MissingGameSystem` class, which
inherits from `StandardError`, and is reraised instead of the
`NoMethodError`.
Starting to improve test coverage for wizard controller

NOTE: This is still failing