Add more output to flaky debug-references test.

test/mjsunit/debug-references.js

 // Copyright 2009 the V8 project authors. All rights reserved.
 // Redistribution and use in source and binary forms, with or without
 // modification, are permitted provided that the following conditions are
 // met:
 //
 //     * Redistributions of source code must retain the above copyright
 //       notice, this list of conditions and the following disclaimer.
 //     * Redistributions in binary form must reproduce the above
 //       copyright notice, this list of conditions and the following
 //       disclaimer in the documentation and/or other materials provided
@@ skipping 80 matching lines @@
 
     // Test some legal references requests.
     testRequest(dcp, '{"handle":' + handle + ',"type":"referencedBy"}', true);
     testRequest(dcp, '{"handle":' + handle + ',"type":"constructedBy"}',
                 true, 2);
 
     // Indicate that all was processed.
     listenerComplete = true;
   }
   } catch (e) {
-    exception = e
+    exception = e;
+    print(e + "\n" + e.stack);
   };
 };
 
 // Add the debug event listener.
 Debug.setListener(listener);
 
 // Test constructor and objects.
 function Point(x, y) { this.x_ = x; this.y_ = y;}
 p = new Point(0,0);
 q = new Point(1,2);
 
 // Enter debugger causing the event listener to be called.
 debugger;
 
 // Make sure that the debug event listener was invoked.
 assertFalse(exception, "exception in listener")
 assertTrue(listenerComplete, "listener did not run to completion");