present proper error page when no master node is available

this page will only been shown for resources that need a master node, detecting the case will be done via a system notification in the server, too, so that users are alerted early.

this might need a few wording changes an a support link, too

fixes #482
in fact, we require one master node.

fixes #482
Merge git://github.com/Graylog2/graylog2-web-interface into develop