[JENKINS-26677] Avoid using ServletException from SlaveComputer since it can...

[JENKINS-26677] Avoid using ServletException from SlaveComputer since it can break calls to SlaveComputer.getChannelToMaster. (#3778)

java.lang.NoClassDefFoundError: javax/servlet/ServletException
	at hudson.util.ProcessTree.get(ProcessTree.java:399)

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

@@ -76,7 +76,6 @@ import org.kohsuke.stapler.interceptor.RequirePOST;
 import javax.annotation.CheckForNull;
 import javax.annotation.Nonnull;
 import javax.annotation.OverridingMethodsMustInvokeSuper;
-import javax.servlet.ServletException;
 import java.io.File;
 import java.io.IOException;
 import java.io.InputStream;
@@ -716,7 +715,7 @@ public class SlaveComputer extends Computer {
     }
 
     @RequirePOST
-    public HttpResponse doDoDisconnect(@QueryParameter String offlineMessage) throws IOException, ServletException {
+    public HttpResponse doDoDisconnect(@QueryParameter String offlineMessage) {
         if (channel!=null) {
             //does nothing in case computer is already disconnected
             checkPermission(DISCONNECT);
@@ -741,11 +740,18 @@ public class SlaveComputer extends Computer {
     }
 
     @RequirePOST
-    public void doLaunchSlaveAgent(StaplerRequest req, StaplerResponse rsp) throws IOException, ServletException {
+    @Override
+    public void doLaunchSlaveAgent(StaplerRequest req, StaplerResponse rsp) throws IOException {
         checkPermission(CONNECT);
             
         if(channel!=null) {
-            req.getView(this,"already-launched.jelly").forward(req, rsp);
+            try {
+                req.getView(this, "already-launched.jelly").forward(req, rsp);
+            } catch (IOException x) {
+                throw x;
+            } catch (/*Servlet*/Exception x) {
+                throw new IOException(x);
+            }
             return;
         }
 
@@ -778,7 +784,7 @@ public class SlaveComputer extends Computer {
     }
 
     @WebMethod(name="slave-agent.jnlp")
-    public HttpResponse doSlaveAgentJnlp(StaplerRequest req, StaplerResponse res) throws IOException, ServletException {
+    public HttpResponse doSlaveAgentJnlp(StaplerRequest req, StaplerResponse res) {
         return new EncryptedSlaveAgentJnlpFile(this, "slave-agent.jnlp.jelly", getName(), CONNECT);
     }