We often send OperationOutcome if there is a problem reading the resource / searching.

Existing Wording:

The CDS Client MUST NOT send any prefetch template key that it chooses not to satisfy. Similarly, if the CDS Client encounters an error while prefetching any data, the prefetch template key MUST NOT be sent to the CDS Service. If the CDS Client has no data to populate a template prefetch key, the prefetch template key MUST have a value of null\. Note that the null result is used rather than a bundle with zero entries to account for the possibility that the prefetch url is a single-resource request\.