It seems forwarding errorText() is required for all KIO errors bcs some error messages aren't build only basing on error code. For example I was getting "Timeout on server\n" instead of "Timeout on server\nbt-2.t-ru.org: Socked operation timed out" that resulted in unnecessary raw height of Tracker list in KTorrent. (https://github.com/KDE/kio/blob/f0456fa7528cd6a68d9cbd97ca5aef19f830b588/src/core/job_error.cpp#L188)
This is not a mistake in HTTP error page processing commit. This is the old bug.