Give MFA gates a more consistent UI

Summary: Depends on D20057. Currently, we show an "MFA" message on one of these and an "Error" message on the other, with different icons and colors. Use "MFA" for both, with the MFA icon / color.

Test Plan: Hit both varations, saw more consistency.

Reviewers: amckinley

Reviewed By: amckinley

Differential Revision: https://secure.phabricator.com/D20059