Submit a form's fields as an EPUB import and normalise the outcome.
Collapses the three failure shapes — transport error, envelope error, and a
handled success: false — into a single error string, so callers only
have to check whether bookId came back.
Submit a form's fields as an EPUB import and normalise the outcome.
Collapses the three failure shapes — transport error, envelope error, and a handled
success: false— into a singleerrorstring, so callers only have to check whetherbookIdcame back.