Adding f-string to validation error which is missing

[luizanao]

FIX #6747

Very minor fix on missing string interpolation.
This will avoid plain non-sense string being returned on API servers implementations.

Before

{
  "object": "error",
  "message": "Unknown model type: {model_type}",
  "type": "BadRequestError",
  "param": null,
  "code": 400
}

After:

{
  "object": "error",
  "message": "Unknown model type: <whatever model type actually is>",
  "type": "BadRequestError",
  "param": null,
  "code": 400
}

[github-actions]

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[DarkLight1337]

Thanks for the fix!