CDS Hooks must support OperationOutcome in prefetch

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.

Proposed wording:

The CDS Client MUST NOT send any prefetch template key that it chooses not to satisfy. If the CDS Client receives an OperationOutcome from the FHIR server when prefetching any data, this SHOULD be included in the prefetch to avoid the CDS Service calling the FHIR server again merely to receive the same error, and also to be consistent with the FHIR specification. However, if the CDS Client encounters any other error while prefetching any data, the prefetch template key MUST NOT be sent to the CDS Service.