fix: fetch_models_task fixed for files and docs added

[kyteinsky]

Changes proposed in this pull request:

• fix fetch_models_task fixed for individual files download
• added docs for fetch_models_task and model dict in set_handlers
• added a custom exception for model download fail since this function can be used independently too

although I'm not too sure of the custom exception, it doesn't serve much purpose except a few pylint ignore comments.

[codecov]

Codecov Report

❌ Patch coverage is 6.66667% with 42 lines in your changes missing coverage. Please review.
✅ Project coverage is 95.11%. Comparing base (9518a89) to head (3976684).
⚠️ Report is 1 commits behind head on main.

Files with missing lines	Patch %	Lines
nc_py_api/ex_app/integration_fastapi.py	4.54%	42 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main     #376      +/-   ##
==========================================
+ Coverage   95.09%   95.11%   +0.01%     
==========================================
  Files          45       45              
  Lines        5360     5361       +1     
==========================================
+ Hits         5097     5099       +2     
+ Misses        263      262       -1     

Files with missing lines	Coverage Δ
nc_py_api/__init__.py	100.00% <ø> (ø)
nc_py_api/_exceptions.py	100.00% <100.00%> (ø)
nc_py_api/ex_app/integration_fastapi.py	25.16% <4.54%> (+0.64%)	⬆️

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