Gracefully handle the error when completing an already completed task

Let's take the example where a task in a workflow is assigned to a group of users, so that all the users in the group can see the task.

If two users open the same task simultaneously (or before the task was completed), only the first one that commits is allowed to execute the task. The other one will get a failure with an error in the browser console, but no error message is displayed to the user (this because the error is not catched in the nuxeo-app.js).

Expected behavior: The second user should see an error message like "The task has been already completed". And no error in the browser console.