Update ConfirmEdit to return Status object on AbortNewAccount hook

Requires core update I6ae34c0 to actually send the message key back
in the error response. On older MediaWiki versions, behavior should
remain unchanged.

Bug: 60008
Change-Id: If9cc08e44d1d2efacf817b8c6cbb2d71a4e8b692
(cherry picked from commit 19dcbb6fa20c2eafab30227c581090d5d6b3d0e5)