Throwing a `CustomNSError` should exit with the custom error code

[natecook1000]

If you throw an error type that conforms to CustomNSError from the run method or a validation closure, the exit code of the program should be the one given by the custom error type's errorCode property.

ArgumentParser version: main branch
Swift version: Apple Swift version 5.3

Checklist

• If possible, I've reproduced the issue using the main branch of this package
• I've searched for existing GitHub issues

Steps to Reproduce

Build and run this command, then use echo $? (or echo $status in fish) to inspect the exit code:

import ArgumentParser
import Foundation

enum CustomError: CustomNSError {
  case error
  
  var errorCode: Int {
    switch self {
    case .error: return 7
    }
  }
}

struct Example: ParsableCommand {
  func run() throws {
    throw CustomError.error
  }
}

Example.main()

Expected behavior

The exit code should be 7.

Actual behavior

It's actually 1, aka EXIT_FAILURE.

[lorentey]

This was a bad idea -- see #269.

[natecook1000]

Ugh, it seems so!