Make return code optional

[emmuhamm]

Description

Fixes #15

See above

Makes return code optional in process manager

Corresponding change in drunc is

Type of change

• Optimization (non-breaking, back-end change that speeds up the code)

Key checklist

• All tests pass (eg. python -m pytest)
• Pre-commit hooks run successfully (eg. pre-commit run --all-files)

Further checks

• Code is commented, particularly in hard-to-understand areas
• Tests added or an issue has been opened to tackle that in the future.
  (Indicate issue here: # (issue))