add exception for unavailable data request

[colligant]

Create class UnavailableDataProductException. This helps users remove products from their query that are not available.

[loicdtx]

Hi @tcolligan4, thanks for the PR; I'll try to look into that this week. Can you send use cases/example queries with unavailable data products that illustrate the exception.
And what about the raise_for_status() method of the request object? Would that be of any use? Like I said, I haven't looked at it in much details yet.