GET /api/data/sessions/:sessionId responds with 422 when the session is not found

[leafty]

GET https://renku-ci-ui-4021.dev.renku.ch/api/data/sessions/flora-thieba-8de7941357db 422 Unprocessable Content

{
    "error": {
        "code": 1422,
        "message": "The session with ID flora-thieba-8de7941357db does not exist."
    }
}

The response should be 404 Not Found.