improved the message when another build is already in progress.

git-svn-id: https://hudson.dev.java.net/svn/hudson/trunk/hudson/main@3833 71c3de6d-444a-0410-be80-ed276b4c234a

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

@@ -659,8 +659,11 @@ public class Queue extends ResourceController {
 
             if(isBlocked) {
                 ResourceActivity r = getBlockingActivity(task);
-                if(r!=null)
+                if(r!=null) {
+                    if(r==task) // blocked by itself, meaning another build is in progress
+                        return "A build is already in progress";
                     return "Blocked by "+r.getDisplayName();
+                }
                 return task.getWhyBlocked();
             }