Support OperationOutcome resource for CDS Hooks prefetch resources

[mrdnctrk]

Version 2 of CDS Hooks specification allows CDS clients to send an OperationOutcome as a prefetch resource if the client fails to prefetch a resource.

There are two main aspects this could affect the HAPI-FHIR's CDS Hooks implementation:

• Since an existing CDS Service implementation may not expect an OperationOutcome as a prefetch resource, it might break such existing implementations. It would be nice to have such implementations fail with a proper error message.
• The auto-prefetch feature may populate a prefetch resource with an OperationOutcome if the prefetching the resource fails.

We need a mechanism to allow CDS Service's to specify how to handle such cases.