Transaction bundle error response should be a single OperationOutcome

[lmsurpre]

Describe the bug
https://www.hl7.org/fhir/http.html#trules

For a failed transaction, the server returns a single OperationOutcome instead of a Bundle.

To Reproduce
See See BundleTest.testTransactionReadDeletedResources()

Expected behavior
A failed transaction request should result in a single OperationOutcome, not a bundle of responses.

[lmsurpre]

Note: we interpret "reject all resources and return an HTTP 400 or 500 type response" to mean it must be either a 400 or 500 we send back.

Touchstone results are still passing.