Expose ErrorHandler protocol; inject errorHandler into Turntable

[Kastet]

• Bumping Xcode to 10.2
• Exposing ErrorHandling protocol and injecting errorHandler into Turntable. This might be useful if custom handling is required, for example, you might want to fail current test if a request wasn't found. This change doesn't introduce breaking changes.

struct ReplayErrorHandler: ErrorHandler {
    func handleTrackNotFound(_ request: Request, playTracksUniquely: Bool) {
        XCTFail("Record not found: \(request)")
    }

    func handleUnknownError() {
        XCTFail("Unknown error")
    }
}

[Kastet]

hey @dmcrodrigues, do you have a chance to review?

[mluisbrown]

Looks good @Kastet 👍 Thanks for the contribution!