Prevent unhandled filesystem exceptions in modify_record

[JSCU-CNI]

This PR prevents unhandled underlying filesystem exceptions when calling a record modifier function. We now check if the resolved_path exists and is a file before attempting to open it.

This is a workaround for the issues discussed in #684.

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 75.54%. Comparing base (f76ac8c) to head (1d92e33).
Report is 1 commits behind head on main.

Additional details and impacted files

@@           Coverage Diff           @@
##             main     #737   +/-   ##
=======================================
  Coverage   75.54%   75.54%           
=======================================
  Files         305      305           
  Lines       26331    26334    +3     
=======================================
+ Hits        19891    19894    +3     
  Misses       6440     6440           

Flag	Coverage Δ
unittests	75.54% <100.00%> (+<0.01%)	⬆️

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