SSO: Better handling of login interruption by wp_login action

The Two-Factor Authentication plugin (core feature, aiming for 4.5) looks into the `wp_login` action, which occurs after our SSO fires, but before `wp_signon` returns the user.

In this case, after our SSO fires, the 2fa plugin asks for a 2fa code. Once submitted, it returns the user to the `https://example.com/wp-login.php?action=jetpack-sso&result=success&user_id=XXXXXX&sso_nonce=XXXXXX` URL, which errors out https://github.com/Automattic/jetpack/blob/master/modules/sso.php#L565

Untested thought: If a current user is set, return out early to the `login_redirect` cookie value?

Ref: https://github.com/georgestephanis/two-factor/issues/84
Marking a vFuture as, assuming 2fa marches on toward an 4.5 (~April 2016) merge, we'll need to be sure this is either handled or compatible with whatever ends up being the final approach.