Improved error propagation from the Python backend to the nvim plugin

[Davidyz]

Makes my life easier to handle different errors.

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 99.48%. Comparing base (671f2c1) to head (9d0a3ea).
Report is 1 commits behind head on main.

Additional details and impacted files

@@            Coverage Diff             @@
##             main     #195      +/-   ##
==========================================
- Coverage   99.48%   99.48%   -0.01%     
==========================================
  Files          21       21              
  Lines        1554     1552       -2     
==========================================
- Hits         1546     1544       -2     
  Misses          8        8              

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