Server-side validation for angular?

Sometimes, client-side only field validation isn't possible. A simple example is field uniqueness checks like making sure a username isn't already in use, but other times there are validations that can only be applied with the submission of the entire form (i.e. violations encoded in an HTTP 400 server response).

What tools or practices have you used to manage server-side validation in you angular applications?

Edit: To be clear, I'm talking about this scenario:

1. Form submits.
2. Backend determines field A is 'invalid', returns HTTP 403, with a json message that indicates which fields are invalid and why.
3. Angular takes in that message and adds annotations to the form elements (e.g .ng-messages).

What tools exist to handle step 3, in part or in whole?