[Chapter 66] Wrong JSON data makes the game crash

https://github.com/amethyst/rustrogueliketutorial/blob/33872fe582f226178436847e1f74eafcbf9c0d1a/book/src/chapter_66.md?plain=1#L123-L134 At that point, `mana_cost` is declared as mandatory on the `Spell` struct in `spell_structs.rs`. Since there is no `mana_cost` field in the JSON file, the game crashes on start when loading the JSON data. I'm not sure how this is meant to be cleaned, as the bug is not fixed in the final state of the code for this chapter.