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