Change CommandError and MessageInfo to public

[icanswiftabit]

Change CommandError and MessageInfo to public.

TL:DR;

The motivation behind it was the desire to react to such error with a beautiful message.

Motivation

In our tool, we use SwiftNIO and ArgumentParser for handling slack commands. At one point a command can fail. For example when a user provides not expect a flag. This will produce CommandError. NIO's mapIfError will bring it to Error type. Hence at this moment, we can only print such error.

CommandError(commandStack: [SlackCommands.PR], parserError: ArgumentParser.ParserError.unknownOption(ArgumentParser.InputOrigin.Element.argumentIndex(0), ArgumentParser.Name.long("some-wrong-argument")))

Not very user friendly :(
But if CommandError and MessageInfo would be public. We could detect if the mapped error is a CommandError and use MessageInfo to print a much nicer message. Such as

Error: Missing expected argument '<project-name>'
Usage: Stats project <project-name> [--repository <repository>]
  See 'Stats project --help' for more information.

This is how we would use it

func handleWebHook(payload: SlackHookPayload) throws -> Future<String> {
    return commandDispatcher.dispatch(payload: payload).mapIfError { error -> String in
        guard let commandError = error as? ArgumentParser.CommandError, let commandType = commandError.commandStack.first.self else {
            return "Command failed with a message: \(error)"
        }
        return MessageInfo(error: commandError, type: commandType).fullText
    }
}

Checklist

• I've added at least one test that validates that my change is working, if appropriate // Not applicable
• I've followed the code style of the rest of the project
• I've read the Contribution Guidelines
• I've updated the documentation if necessary // Not applicable

[natecook1000]

You can currently call message(for: error) or fullMessage(for: error) on your command types to convert an error thrown from ArgumentParser into a human-readable string. We aren't ready to expose CommandError or MessageInfo as public API yet — would you mind opening an issue with your use case, instead?

[icanswiftabit]

Ok, these methods do help in our case. I can easily recreate command from SlackHookPayload. But I'm sure there is a case where knowing about CommandError would be great.
Ahh, a contribution was so close. Thanks for the help :) Here is the issue #199