Fix delegation when validating the form without filling the mandatory field

Steps to follow to reproduce: As an Administrator user:

1. Start a serial workflow with a least one participant
2. Go to the Workflow tab
3. Click on the Delegate Task button
4. It opens a fancybox
5. Click on the Delegate button without filling the Users field
6. The form is submitted and the workflow tab is displayed

On this page, if you click on the Delegate Task button, it does not display the fancybox, maybe because the current user is not the current assignee any more (incoherent state?)

We need at least to fix the validation of mandatory fields in the fancybox