improved error detection.

git-svn-id: https://hudson.dev.java.net/svn/hudson/trunk/hudson/main@3852 71c3de6d-444a-0410-be80-ed276b4c234a

core/src/main/resources/hudson/model/Computer/index.jelly

@@ -22,6 +22,13 @@
       <j:choose>
         <j:when test="${it.jnlpAgent}">
           <j:choose>
+            <j:when test="${app.slaveAgentPort==-1}">
+              <img src="${rootURL}/images/32x32/error.gif"/>
+              <span class="error">
+                TCP port for JNLP slave agentsis is disabled.
+                <a href="${rootURL}/configure">Go to system config screen and change it</a>.
+              </span>
+            </j:when>
             <j:when test="${it.offline &amp;&amp; !it.temporarilyOffline}">
               <l:isAdmin>
                 <p>