gmail auth: return proper HTTP response to avoid qwebengine error
Test Plan:
"KGAPI2::AuthWidgetPrivate::webviewFinished: Failed to load ..."
no longer shows when logging into gmail
Reviewers: dvratil
Reviewed By: dvratil
Subscribers: kde-pim
Tags: KDE PIM
Differential Revision: https://phabricator.kde.org/D14845