Answer
This is confirmed current platform behavior rather than a configuration gap — response messages for any status code above 299 are overridden before reaching the workflow, and there's currently no way to change this and surface the full original response body to the user; if the specific embedded error detail matters for troubleshooting, you'll need to reproduce the call outside the workflow (in Postman or similar) to see it, rather than expecting the workflow's own error display to carry it through.