Unhelpful Error Message If Conditional Reference Points to Multiple Records.

Describe the bug
If you have a conditional reference that returns multiple results, then the error message indicates that the reference does not resolve to a resource.

FHIR Version?
All

Data provider?
All

To Reproduce
Steps to reproduce the behavior:

1. Create, for instance, duplicate organizations with the same identifier.
2. Attempt to create, for instance, an encounter object referencing that organization through a conditional reference.
3. Receive error message.

Expected behavior
Receive a message indicating that the conditional reference was ambiguous.

b'{"resourceType":"OperationOutcome","id":"a1f74190-04f6-4369-83b6-0141a32b440d","issue":[{"severity":"error","code":"invalid","diagnostics":"Given conditional reference \'Organization?identifier=https://github.com/synthetichealth/synthea|3bd5eda0-16da-3ba5-8500-4dfd6ae118b8\' resolved to multiple resources."}]}'
Actual behavior
Receive a message indicating that the conditional reference does not resolve to a resource.

b'{"resourceType":"OperationOutcome","id":"a1f74190-04f6-4369-83b6-0141a32b440d","issue":[{"severity":"error","code":"invalid","diagnostics":"Given conditional reference \'Organization?identifier=https://github.com/synthetichealth/synthea|3bd5eda0-16da-3ba5-8500-4dfd6ae118b8\' does not resolve to a resource."}]}'

Start by reproducing the conditional-reference case described in the issue: create duplicate Organizations with the same identifier, then reference one from an Encounter. Trace the code that builds the OperationOutcome and verify that ambiguous matches produce the expected multiple-resources message rather than the unresolved-reference message.