Fix RequestAccountPage::confirmEmailToken()

Follows-up r33333 (41d48826812b in mediawiki/core), which made a
backward-incompatible change to User::confirmEmail().

Also removed the reference to the 'confirmemail_error' message (as
that commit did in core) and changed the code to pass $title directly
to OutputPage::returnToMain().

Change-Id: I22c3e29b5f5b46c69dad39d6930bc29546162940