Potential fix for code scanning alert no. 48: Information exposure through an exception

[frode-aarstad]

Potential fix for https://github.com/equinor/ert/security/code-scanning/48

Use a generic, non-sensitive error message for both:

1. experiment_state.status.message
2. API response {"error": ...}

Keep detailed diagnostics in server logs using logger.exception(...) inside the except block. This preserves functionality (request still fails and is reported as failed) without exposing internal exception details to clients.

In src/ert/dark_storage/endpoints/experiment_server.py, update only the except Exception as e: block in start_experiment:

• Introduce a constant/local string like "Could not start experiment due to an internal error."
• Set experiment_state.status.message to that string (instead of interpolating e)
• Return JSONResponse({"error": generic_message}, status_code=501) (instead of interpolating e)
• Keep server-side logging with .exception(...) so developers still get stack traces.

No new imports or dependencies are required.

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[tbwiik]

lgtm:) (after check is fixed)

[tbwiik]

Flaky gh actions