Add extra info if the slave can not restartc correctly.

If the service can not reliably restart itself add link to the wiki with
some information about how to fix it.

core/src/main/java/jenkins/slaves/restarter/WinswSlaveRestarter.java

@@ -51,7 +51,9 @@ public class WinswSlaveRestarter extends SlaveRestarter {
         // so it's possible that we end up in the situation where jenkins-slave.exe doesn't support
         // this command. If that is the case, there's nothing we can do about it.
         int r = exec("restart!");
-        throw new IOException("Restart failure. '"+exe+" restart' completed with "+r+" but I'm still alive");
+        throw new IOException("Restart failure. '"+exe+" restart' completed with "+r+" but I'm still alive!  "
+                               + "See https://wiki.jenkins-ci.org/display/JENKINS/Distributed+builds#Distributedbuilds-Windowsslaveserviceupgrades"
+                               + " for a possible explanation and solution");
     }
 
     private static final Logger LOGGER = Logger.getLogger(WinswSlaveRestarter.class.getName());