Don't generate stacktrace in TaskCancelledException

Author: iverase

server/src/main/java/org/elasticsearch/tasks/TaskCancelledException.java

@@ -27,6 +27,11 @@ public TaskCancelledException(StreamInput in) throws IOException {
         super(in);
     }
 
+    @Override
+    public Throwable fillInStackTrace() {
+        return this;  // this exception doesn't imply a bug, no need for a stack trace
+    }
+
     @Override
     public RestStatus status() {
         // Tasks are typically cancelled at the request of the client, so a 4xx status code is more accurate than the default of 500 (and