explicitly ignore iopub messages not associated with a cell in the notebook

An error would previously be raised whenever iopub traffic arrived that
did not originate with the notebook (e.g. pylab welcome, or qtconsole output).

This error was not in a block that would actually cause problems, so actual
functionality was not affected.

IPython/frontend/html/notebook/static/js/notebook.js

@@ -726,6 +726,11 var IPython = (function (IPython) {
         // console.log(reply);
         var msg_type = reply.header.msg_type;
         var cell = this.cell_for_msg(reply.parent_header.msg_id);
+        if (!cell){
+            // message not from this notebook
+            console.log("Received IOPub message not caused by one of my cells");
+            return;
+        }
         var output_types = ['stream','display_data','pyout','pyerr'];
         if (output_types.indexOf(msg_type) >= 0) {
             this.handle_output(cell, msg_type, content);