How to declare Error in Swift

Issue #704

We used to declare enum that conforms to Error, but any type like struct or class can conform to Error as well.

enum NetworkError: Error {
    case failToCreateRequest
    case failToParseResponse
    case failToReachServe
}

struct DetailError: Error {
    let networkError: Error
    let createdAt: Date
    let tag: String
}

final class TrackError: Error {
    let trackId: String
    let detailError: DetailError
    let trackSession: String

    init(trackId: String, detailError: DetailError, trackSession: String) {
        self.trackId = trackId
        self.detailError = detailError
        self.trackSession = trackSession
    }
}

let networkError = NetworkError.failToCreateRequest
let detailError = DetailError(networkError: networkError, createdAt: Date(), tag: "0.1")
let trackError = TrackError(trackId: "AB-01", detailError: detailError, trackSession: "101")