Simplify error handling for errors in sending sync messages from js

IPython/html/static/widgets/js/widget.js

@@ -396,10 +396,9 define(["widgets/js/manager",
                     }
                 }
                 that.comm.send({method: 'backbone', sync_data: state, buffer_keys: buffer_keys}, callbacks, {}, buffers);
-            }).catch(utils.reject("Couldn't send widget sync message", true))
-                .catch(function(error) {
+            }).catch(function(error) {
                     that.pending_msgs--;
-                    return error;
+                return (utils.reject("Couldn't send widget sync message", true))(error);
                 });
         },