Grancefully handle SSO redirections on Ajax requests

On some SSO systems, when the SSO session is expired,
the system intercept HTTP requests and send a 302 "Found" HTTP code to the login page.
This patch handle this case in Roundcube Ajax requests, to redirect to the SSO login page.
Note that request.status don't have the 302 code (at least on Firefox), so we have to check the response headers for a Location field and redirect manually.