Handle unopenable files consistently in config

[eivindjahren]

Issue
Resolves #9681

• PR title captures the intent of the changes, and is fitting for release notes.
• Added appropriate release note label
• Commit history is consistent and clean, in line with the contribution guidelines.
• Make sure unit tests pass locally after every commit (git rebase -i main --exec 'pytest tests/ert/unit_tests -n logical -m "not integration_test"')

When applicable

• When there are user facing changes: Updated documentation
• New behavior or changes to existing untested code: Ensured that unit tests are added (See Ground Rules).
• Large PR: Prepare changes in small commits for more convenient review
• Bug fix: Add regression test for the bug
• Bug fix: Create Backport PR to latest release

[codspeed-hq]

CodSpeed Performance Report

Merging #9682 will not alter performance

_{Comparing eivindjahren:fix_read_file (071daa0) with main (71e38f2)}

Summary

✅ 24 untouched benchmarks

[codecov-commenter]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 91.74%. Comparing base (383d645) to head (071daa0).
Report is 5 commits behind head on main.

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #9682      +/-   ##
==========================================
- Coverage   91.74%   91.74%   -0.01%     
==========================================
  Files         426      426              
  Lines       26518    26553      +35     
==========================================
+ Hits        24328    24360      +32     
- Misses       2190     2193       +3     

Flag	Coverage Δ
cli-tests	39.66% <60.86%> (-0.04%)	⬇️
everest-models-test	34.18% <43.47%> (-0.04%)	⬇️
gui-tests	74.32% <60.86%> (+2.37%)	⬆️
integration-test	38.01% <43.47%> (-0.06%)	⬇️
performance-tests	51.70% <60.86%> (+0.05%)	⬆️
test	39.53% <52.17%> (-0.06%)	⬇️
unit-tests	74.25% <100.00%> (-0.02%)	⬇️

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