Show message that slave has been already launched instead of source not found

core/src/main/java/hudson/slaves/SlaveComputer.java

@@ -447,7 +447,7 @@ public class SlaveComputer extends Computer {
 
     public void doLaunchSlaveAgent(StaplerRequest req, StaplerResponse rsp) throws IOException, ServletException {
         if(channel!=null) {
-            rsp.sendError(HttpServletResponse.SC_NOT_FOUND);
+            req.getView(this,"already-launched.jelly").forward(req, rsp);
             return;
         }
 

core/src/main/resources/hudson/slaves/SlaveComputer/already-launched.jelly

+<?jelly escape-by-default='true'?>
+<j:jelly xmlns:j="jelly:core" xmlns:st="jelly:stapler" xmlns:d="jelly:define" xmlns:l="/lib/layout" xmlns:t="/lib/hudson" xmlns:f="/lib/form" xmlns:i="jelly:fmt">
+  <l:layout title="${it.displayName}">
+    <st:include page="sidepanel.jelly" />
+    <l:main-panel>
+      The slave has been already launched.
+    </l:main-panel>
+  </l:layout>
+</j:jelly>