Diagnostic toString.

Author: jglick

src/main/java/org/jenkinsci/plugins/workflow/log/TaskListenerDecorator.java

@@ -290,6 +290,10 @@ private CloseableTaskListener(TaskListener mainDelegate, TaskListener closeDeleg
             ((AutoCloseable) closeDelegate).close();
         }
 
+        @Override public String toString() {
+            return "CloseableTaskListener[" + mainDelegate + " / " + closeDelegate + "]";
+        }
+
     }
 
 }