fix(task_manager): allow tasks to sometimes fail via return value

[mini-bomba]

🔗 Linked issue

fixes #22

❓ Type of change

• 🐞 Bug fix (a non-breaking change that fixes an issue)
• 👌 Enhancement (improving an existing functionality like performance)
• ✨ New feature (a non-breaking change that adds functionality)
• ⚠️ Breaking change (fix or feature that would cause existing functionality to change)

📚 Description

i've basically just merged the 3 possible promise return types into one promise type with a union result type.
this allows async tasks submitted via the task manager to have both success and failure return branches; before this PR async tasks could only either always fail via return value, or always return success via return value and fail by throwing an exception - using task.error() (where task is the parameter given to task callbacks) was basically impossible in actual async applications.

i've also added a typecheck-only test that verifies that submitting a non-deterministic async task is possible.
this test will never fail during npm run test checks, but may emit errors in npm run typecheck instead

📝 Checklist

• I have read the contribution guide.
• I have linked an issue or discussion.
• I have updated the documentation accordingly.

[thetutlage]

Thanks 👍