makes Error extensible

[dnadoba]

Allows user code to add custom error codes

Motivation:

A user should be able to easily define and use custom application errors

Modifications:

• Error is now a struct instead of an enum with two stored properties code and message
• Error.Code is a new struct which is a thin wrapper around a UInt32 which allows to add custom error codes through extensions

Result:

A user can now define custom errors by writing two extensions:

extension Error.Code {
    static let myCustomError = Self(rawValue: 0x00000301)
}
extension Error {
    static func myCustomError(message: String) -> Self { .init(code: .myCustomError, message: message ) }
}

Custom Errors can be used in the same places as predefined errors and behave the same too.

[OlegDokuka]

LGTM