Add more output to flaky debug-references test.

R=vogelheim@chromium.org

Review URL: https://codereview.chromium.org/1006873002

Cr-Commit-Position: refs/heads/master@{#27181}

test/mjsunit/debug-references.js

@@ -98,7 +98,8 @@ function listener(event, exec_state, event_data, data) {
     listenerComplete = true;
   }
   } catch (e) {
-    exception = e
+    exception = e;
+    print(e + "\n" + e.stack);
   };
 };