server: Disallow sending events to clients after posting an error

Until now, we haven't done anything to prevent sending additional
events to clients after posting an error.

Acked-by: Daniel Stone <daniels@collabora.com>
Signed-off-by: Derek Foreman <derekf@osg.samsung.com>
Reviewed-by: Pekka Paalanen <pekka.paalanen@collabora.co.uk>