Make Zuul alert messages prominent.

With any luck, very hard to miss indeed.

Change-Id: Icdf7ee2fd6de5e2e2e7916ab662887110484bfca
Reviewed-on: https://review.openstack.org/19990
Reviewed-by: Jeremy Stanley <fungi@yuggoth.org>
Approved: Clark Boylan <clark.boylan@gmail.com>
Reviewed-by: Clark Boylan <clark.boylan@gmail.com>
Tested-by: Jenkins