Function isErrorInBody

Detect an EP API "error-in-body" response.

Some feed endpoints return HTTP 200 but with a JSON body that contains an error field and no data array. Example:

{
  "@id": "https://data.europarl.europa.eu/eli/dl/…",
  "error": "404 Not Found from POST …",
  "@context": { "error": { … } }
}
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This function returns true when the parsed response has this shape, allowing callers to convert it to an empty feed response.