Feature/improve error handling

[bizz84]

This PR streamlines error handling. See related issue: #122

This makes it much easier to check all possible errors that can happen during a purchase:

• clientInvalid: Not allowed to make the payment
• paymentCancelled: The user has cancelled the payment
• paymentInvalid: The purchase identifier was invalid
• paymentNotAllowed: In-app purchases have been disabled for the device
• storeProductNotAvailable: The product is not available in the current storefront
• cloudServicePermissionDenied: Access to cloud service information is not allowed
• cloudServiceNetworkConnectionFailed: Could not connect to the network
• unknown: anything else

This is in line with: https://developer.apple.com/reference/storekit/skerror.code

This is an API breaking change as the PurchaseError type has been removed in favour of SKError. Clients can now make purchases and check errors like this:

SwiftyStoreKit.purchaseProduct("com.musevisions.SwiftyStoreKit.Purchase1", atomically: true) { result in
    switch result {
    case .success(let product):
        print("Purchase Success: \(product.productId)")
    case .error(let error):
        switch error.code {
        case .unknown: print("Unknown error. Please contact support")
        case .clientInvalid: print("Not allowed to make the payment")
        case .paymentCancelled: break
        case .paymentInvalid: print("The purchase identifier was invalid")
        case .paymentNotAllowed: print("The device is not allowed to make the payment")
        case .storeProductNotAvailable: print("The product is not available in the current storefront")
        case .cloudServicePermissionDenied: print("Access to cloud service information is not allowed")
        case .cloudServiceNetworkConnectionFailed: print("Could not connect to the network")
        }
    }
}