Support Exit codes from thrown CustomNSError conformers

[SergeyPetrachkov]

Introduce support for CustomNSError protocol conformers. Previously executable targets would exit with code 1, but it seems like a good idea to exit with a code provided by the user's custom error type.

I've added a sample executable target, and a few tests to cover things up.

This PR references #243 and #230

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(no need i guess)

[SergeyPetrachkov]

@swift-ci please test

[natecook1000]

Thanks for this contribution, @SergeyPetrachkov! I think we can test this without adding the separately compiled example — can you add your tests for handling CustomNSError types to ExitCodeTests.swift?

[SergeyPetrachkov]

Thanks for this contribution, @SergeyPetrachkov! I think we can test this without adding the separately compiled example — can you add your tests for handling CustomNSError types to ExitCodeTests.swift?

Yes, sure, I'll do it

[SergeyPetrachkov]

@natecook1000 I've updated the PR according to your notes. Take a look when you have time.

[natecook1000]

@swift-ci Please test