Throw exceptions on error

[nbarbettini]

If the API returns a 4xx or 5xx status code, the SDK must throw an exception with the error information.

[olexiylebedyev-okta]

In Swift it is not possible to throw in async calls, but we have error handling. Use func handleError(_ error: OktaError) in AuthenticationClientDelegate

Supported errors:

public enum OktaError: Error {
    case errorBuildingURLRequest
    case connectionError(Error)
    case emptyServerResponse
    case responseSerializationError(Error)
    case serverRespondedWithError(OktaAPIErrorResponse)
    case authenicationStatusNotSupported(AuthStatus)
    case unexpectedResponse
}