Fix: Use FileNotFoundError when no file found rather than FileExistsError

[HCookie]

When no file found to read from, a FileExistsError was being thrown,
Would FileNotFoundError be more apt?

[FussyDuck]

All committers have signed the CLA.

[sandorkertesz]

I agree. I see now the test has been updated.

[codecov-commenter]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 90.92%. Comparing base (09c6cba) to head (3d23654).

Additional details and impacted files

@@           Coverage Diff            @@
##           develop     #479   +/-   ##
========================================
  Coverage    90.92%   90.92%           
========================================
  Files          139      139           
  Lines         9532     9532           
  Branches       829      829           
========================================
  Hits          8667     8667           
  Misses         713      713           
  Partials       152      152           

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