Issue Description:

Swift Package Manager utilities classes ArgumentParserError and ArgumentConversionError are not conforming to LocalizedError protocol which means the property localizedError returns a technical error message like:

Utility.ArgumentParserError error 1.

This can be quickly fixed in project with something similar to:

extension ArgumentParserError: LocalizedError {
    public var errorDescription: String? {
        return description
    }
}

This change would lead to a more user friendly error message, like:

unknown option XYZ; use --help to list available options

I think LocalizedError protocol should be used for both ArgumentParserError and ArgumentConversionError inside SPM's code directly.