Improved error propagation from cmd jobrunner

[Davidyz]

The plenary.job API was replaced by vim.system, which provides a much simpler interface to work with. This makes the error propagation clearer, avoiding potential loss of error messages.

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 99.72%. Comparing base (4232d9c) to head (bdb252d).
⚠️ Report is 1 commits behind head on main.

Additional details and impacted files

@@           Coverage Diff           @@
##             main     #285   +/-   ##
=======================================
  Coverage   99.72%   99.72%           
=======================================
  Files          25       25           
  Lines        1837     1837           
=======================================
  Hits         1832     1832           
  Misses          5        5           

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