뭐가 잘못됐는지 말해주는 에러

import Foundation

struct Capture: Codable { let id: String; var text: String }

enum InboxError: Error, CustomStringConvertible {
    case containerMissing(group: String)
    case unreadable(file: String, reason: String)

    var description: String {
        switch self {
        case .containerMissing(let group):
            return "App Group container \(group) is not reachable — check the entitlement"
        case .unreadable(let file, let reason):
            return "\(file) could not be read: \(reason)"
        }
    }
}

func requireContainer(_ url: URL?, group: String) throws(InboxError) -> URL {
    guard let url else { throw .containerMissing(group: group) }
    return url
}

struct Listing<Item> {
    var items: [Item]
    var unreadable: [InboxError]      // the caller cannot accidentally not look
}

func listCaptures(in folder: URL) throws(InboxError) -> Listing<Capture> {
    let files: [URL]
    do {
        files = try FileManager.default.contentsOfDirectory(at: folder, includingPropertiesForKeys: nil)
    } catch {
        // A folder that cannot be listed is not an empty inbox.
        throw .unreadable(file: folder.lastPathComponent, reason: "\(error)")
    }
    var listing = Listing<Capture>(items: [], unreadable: [])
    for file in files where file.pathExtension == "json" {
        do {
            listing.items.append(try JSONDecoder().decode(Capture.self, from: Data(contentsOf: file)))
        } catch {
            listing.unreadable.append(.unreadable(file: file.lastPathComponent, reason: "\(error)"))
        }
    }
    return listing
}