contract says can't return null.

git-svn-id: https://hudson.dev.java.net/svn/hudson/trunk/hudson/main@4238 71c3de6d-444a-0410-be80-ed276b4c234a

core/src/main/java/hudson/model/Slave.java

@@ -218,7 +218,8 @@ public final class Slave implements Node, Serializable {
 
     public ClockDifference getClockDifference() throws IOException, InterruptedException {
         VirtualChannel channel = getComputer().getChannel();
-        if(channel==null)   return null;   // can't check
+        if(channel==null)
+            throw new IOException(getNodeName()+" is offline");
 
         long startTime = System.currentTimeMillis();
         long slaveTime = channel.call(new Callable<Long,RuntimeException>() {