system-level endpoint returns HTML 404 instead of FHIR OperationOutcome

[jmandel]

Bug

POST /r4/$closure returns an HTML page (Cannot POST /r4/$closure) instead of a FHIR OperationOutcome. All FHIR server error responses must be OperationOutcome resources.

Repro

curl -s -X POST 'https://tx-dev.fhir.org/r4/$closure' \
  -H 'Content-Type: application/fhir+json' \
  -H 'Accept: application/fhir+json' \
  -d '{"resourceType":"Parameters","parameter":[{"name":"name","valueString":"test-closure"}]}'
# Returns: HTML body with "<pre>Cannot POST /r4/$closure</pre>"
# Expected: FHIR OperationOutcome (either proper $closure response or "not-supported")

Spec reference

R4 section 2.21.0.1: All error responses SHALL be OperationOutcome resources.

[grahamegrieve]

All FHIR server error responses must be OperationOutcome resources.

that is not actually true; only the interactions documented in the CapabilityStatement must have FHIR responses