把这段代码封装到一个函数包里

当然可以，我们可以创建一个名为`FibonacciErrorDetection`的函数包，包含一个名为`findFirstIncorrectFibonacci`的函数，该函数接收一个整数参数`nGuess`，然后执行上述的Fibonacci数列错误检测逻辑。以下是这个函数包的实现：

import Foundation
import SwiftNumbers

enum FibonacciErrorKind {
    case noneFound
    case errorDetected
}

func findFirstIncorrectFibonacci(maxIterations: Int = 1000) -> FibonacciErrorKind? {
    func fibonacci(_ n: Int) -> Double {
        let phi = (1.0 + sqrt(Double(5))) / 2.0
        return pow(phi, Double(n)) / sqrt(Double(5))
    }

    var nGuess = 0
    var previousFib: Double?
    
    while nGuess <= maxIterations {
        let fibNum = fibonacci(nGuess)
        let expectedFib = floor(phi.power(nGuess) / sqrt(Double(5))).roundedValue

        if let prevFib = previousFib, abs(fibNum - prevFib) > 1e-10 {
            return .errorDetected(n: nGuess, actual: fibNum, expected: expectedFib)
        }

        previousFib = fibNum
        nGuess += 1
        
        if isCloseEnough(to: fibNum, expected: expectedFib) {
            return .noneFound
        }
    }

    return nil // 如果未找到错误，返回nil
}

// 辅助函数判断两个浮点数是否近似相等
private func isCloseEnough(to actual: Double, expected: Int) -> Bool {
    return abs(actual - Double(expected)) < 1e-10
}

// 使用示例
if let errorKind = findFirstIncorrectFibonacci() {
    switch errorKind {
    case .errorDetected(let n, let actual, let expected):
        print("Error first occurs for n=\(n), actual: \(actual), expected: \(expected)")
    case .noneFound:
        print("No error found within the given iterations.")
    default:
        assertionFailure("Unexpected error kind")
    }
} else {
    print("No error found after all iterations.")
}

现在你可以通过`findFirstIncorrectFibonacci()`函数来检测Fibonacci数列中的错误，并根据返回的结果进行相应的处理。