Make DataResponse.payload be non-optional

[sffc]

If the data isn't present, an error should be returned. It's likely we don't need the payload to be an Option.

[sffc]

I investigated this; the only case where we set None is in draft PR #4607

[sffc]

As discussed in #58, we plan to add a can_load function to the DataProvider traits, meaning work can be done on this issue.