Add an API for converting an error to an exit code

[natecook1000]

Description

This provides a method on all ParsableArguments/ParsableCommand types that lets you retrieve the exit code associated with an error.

Detailed Design

1. Add an exitCode(for:) method that converts an error to an ExitCode instance.

extension ParsableArguments {
  /// Returns the exit code for the given error.
  ///
  /// - Parameter error: An error to generate a message for.
  /// - Returns: The exit code for the `error`.
  public static func exitCode(for error: Error) -> ExitCode
}

2. Upgrade the ExitCode type to be a bit more useful, making it RawRepresentable and Hashable. The represented code is now available through the rawValue property.

Documentation Plan

I included symbol documentation for the new APIs.

Test Plan

I added unit tests for the exitCode(for) method and converted the integration test helper to use ExitCodes instead of plain Int32s.

Source Impact

No source impact or deprecations.

Checklist

• I've added at least one test that validates that my change is working, if appropriate
• 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

[miguelangel-dev]

I think you should use directly ExitCode as a type for AssertExecuteCommand - to force you to keep tests with the same semantic and if it is the fail-output for the execution, the type should be always the same.

If you use Int32 you will be less strict, and some bugs can be introduced. wdut?

Suggested change

	exitCode: Int32 = ExitCode.success.rawValue,
	exitCode: ExitCode = .success,

[natecook1000]

👍 I was resisting this for some reason. Thanks!