Improve error message when specifying unknown data format

[chaoran-chen]

Currently, when calling an entpoint with an unknown data format (example), the error message is, for example:

{
    "type": "about:blank",
    "title": "Not Acceptable",
    "status": 406,
    "detail": "Acceptable representations: [text/tab-separated-values, application/json, text/csv].",
    "instance": "/open/v2/sample/aggregated"
}

It would be better to list the allowed values for the dataFormat parameter.