Returns all errors for a completed Multi FileFeed import as a unified,
node-centric payload. Each errored node in the workflow is represented as
an entry in node_errors with per-list error details and presigned CSV
download URLs.

For v1 flowgraphs (import node), errors are synthesized from the
flowgraph run's validation_result. For v2 flowgraphs
(validate-against-template node), errors come directly from the node
run's details.