Following this answer, I have tried restarting a Jenkins instance using:
(jenkins_url)/safeRestart
and
(jenkins_url)/restar
If your Jenkins instance's servlet container does not support a web application restarting its own context, then this plugin cannot provide restart functionality, and the “Restart Safely” link will not appear.
I'm using Apache Tomcat v7.0.64 and after installing SafeRestart Plugin I got the same message as you had.
I think it would be much simpler for you if you just run Jenkins with the embedded container and the service wrapper