Change `CommandError` and `MessageInfo` to `public`

[icanswiftabit]

As a follow up to pull request #198

TL:DR;

The motivation behind it was the desire to react to such an 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
    }
}