Fix a subtle async bug in processing comm messages.

Basically, message processing did not wait for any promises a comm message handler may return to resolve, because comms were not set up to resolve return values of message handlers.

notebook/static/services/kernels/comm.js

@@ -128,12 +128,9 @@ define([
         }
 
         this.comms[content.comm_id] = this.comms[content.comm_id].then(function(comm) {
-            try {
-                comm.handle_msg(msg);
-            } catch (e) {
-                console.log("Exception handling comm msg: ", e, e.stack, msg);
-            }
-            return comm;
+            return (Promise.resolve(comm.handle_msg(msg))
+                .catch(utils.reject('Exception handling comm message'))
+                .then(function() {return comm;}));
         });
         return this.comms[content.comm_id];
     };
@@ -193,7 +190,7 @@ define([
         var callback = this['_' + key + '_callback'];
         if (callback) {
             try {
-                callback(msg);
+                return callback(msg);
             } catch (e) {
                 console.log("Exception in Comm callback", e, e.stack, msg);
             }
@@ -201,7 +198,7 @@ define([
     };
     
     Comm.prototype.handle_msg = function (msg) {
-        this._callback('msg', msg);
+        return this._callback('msg', msg);
     };
     
     Comm.prototype.handle_close = function (msg) {