Work out reconciliation process for retrieve providers

[sliver007]

"Inject prefetch resources some other way, like parameters in CQL", but that breaks a whole bunch of ecosystem stuff. null = unknown empty empty = known empty otherwise = known non-empty.

That obviously has potentially far-reaching implications for engine behavior. But I think a first pass is not too bad. Null dataProvider result = new CqlException("DataProvider was unable to determine if retrieve XXX was null or empty. Please ensure the evaluation environment has all data necessary to make that determination")

Then in the context of prefetch, we can fall back. In the context of using a REST provider directly, something went horribly wrong.