Swift：Errors thrown from here are not handled because the enclosing catch is not exhaustive

在学习 Swift 错误处理的时候，官方给出的 do-catch 例子如下：

...
...

let favoriteSnacks = [
    "Alice": "Chips",
    "Bob": "Licorice",
    "Eve": "Pretzels",
]
func buyFavoriteSnack(person: String, vendingMachine: VendingMachine) throws {
    let snackName = favoriteSnacks[person] ?? "Candy Bar"
    try vendingMachine.vend(itemNamed: snackName)
}

var vendingMachine = VendingMachine()
vendingMachine.coinsDeposited = 8
do {
    try buyFavoriteSnack(person: "Alice", vendingMachine: vendingMachine)
} catch VendingMachineError.invalidSelection {
    print("Invalid Selection.")
} catch VendingMachineError.outOfStock {
    print("Out of Stock.")
} catch VendingMachineError.insufficientFunds(let coinsNeeded) {
    print("Insufficient funds. Please insert an additional \(coinsNeeded) coins.")
}
// Prints "Insufficient funds. Please insert an additional 2 coins."

但是亲自上手敲代码的时候，却总是在 “do” 闭包中 try 语句上报错：

“Errors thrown from here are not handled because the enclosing catch is not exhaustive”

大体意思是说这个 do-catch 是不完整的。这时候需要再加上一个空的 catch 语句用于关闭 catch。

do {
    try buyFavoriteSnack(person: "Alice", vendingMachine: vendingMachine)
} catch VendingMachineError.invalidSelection {
    print("Invalid Selection.")
} catch VendingMachineError.outOfStock {
    print("Out of Stock.")
} catch VendingMachineError.insufficientFunds(let coinsNeeded) {
    print("Insufficient funds. Please insert an additional \(coinsNeeded) coins.")
} catch { // 加入一个空的catch，用于关闭catch。否则会报错：Errors thrown from here are not handled because the enclosing catch is not exhaustive

}