OutOfProcessReferenceResolver.ResponseV2.failure(_:)

A response with human-readable information about the problem that the external link resolver encountered while resolving the requested link or symbol.

case failure(OutOfProcessReferenceResolver.ResponseV2.DiagnosticInformation)

Discussion

Note
DocC doesn’t display any diagnostics about failed OutOfProcessReferenceResolver.RequestV2.symbol(_:) requests because they wouldn’t be actionable; because they’re entirely automatic—based on unique identifiers in the symbol graph files—rather than authored references in the content.
Your external link resolver still needs to send a response to each request but it doesn’t need to include details about the failure when responding to a OutOfProcessReferenceResolver.RequestV2.symbol(_:) request.