Show a better error message when the polkit agent is missing.
We are getting an error report once in a while because of a system that does not have a running/installed polkit agent. We need to show a clear message about that problem and maybe a link to a help page to solve that issue.
(from redmine: created on 2014-09-29, closed on 2014-12-18, relates #6149 (closed), relates #6127 (closed), relates #6123 (closed), relates #6076, relates #6101, relates #6043 (closed), relates #6125 (closed))