[Chapter 66] Wrong JSON data makes the game crash

[PierreGuyot]

rustrogueliketutorial/book/src/chapter_66.md

Lines 123 to 134 in 33872fe

	```json
	"spells" : [
	{
	"name" : "Zap",
	"effects" : {
	"ranged" : "6",
	"damage" : "5",
	"particle_line" : "▓;#00FFFF;200.0"
	}
	}
	]
	```

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.
The final state of the code for this chapter does include this mana_cost field.