check $?.success? after waitpid

[obfusk]

🔤 Polyglot PR

Fixes #144.

Type of change

• [ ] Docs update (changes to the readme or a site page, no code changes)
• [ ] Ops wrangling (automation or test improvements)
• [x] Bug fix (non-breaking change that fixes an issue)
• [ ] New feature (non-breaking change that adds functionality)
• [ ] Breaking change (fix or feature that would cause existing functionality to not work as expected)
• [ ] Sweet release (needs a lot of work and effort)
• [ ] Something else (explain please)

Checklists

• [ ] If modifying code, at least one test has been added to the suite

[obfusk]

cc @eighthave