Previously, when the job reported a failure, a generic error message provided by QtWebEngine was displayed. Now, WebEnginePartKIOHandler makes use of KIO::StoredTransferJob::errorString to display a more informative message.
Some ioslave, in particular the man ioslave (at least after commit d5f2beb2c13c8b3a202b4979027ad5430f007705) leave the error string empty and put the error message inside data(). Because of this, if errorString is empty and data is not empty, data is used.