Improve UX for permission errors in storage

[jonathan-eq]

This commit:

• Improves the error message displayed when the dark storage server does not have access to the storage path.
• Makes the dark storage server return a response with status code 401 - unauthorized when the get_ensemble_record endpoint fails due to PermissionError.
• Makes the failed message in LegacyEnsemble._evaluate_inner omit stacktrace when it failed due to PermissionError, making it shorter and more consise.

Issue
Resolves #9493

(Screenshot of new behavior in GUI if applicable)

• 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