I spent a day trying to figure out how to get ember-bootstrap to handle server-side (HTTP 422) validation errors. I tried the three supported validation packages (two of which don't work with the newest version of Ember.js), and finally began to understand that ember-cp-validations has nothing to do with server-side errors.
So, taking inspiration from ember-bootstrap-ds-error-validations, which causes errors when combined with another validation package, I was able to put something fairly simple in place.
I'll be honest, I don't entirely know what the app/components/bs-form.js file is doing (this was taken from ember-bootstrap-cp-validations I think), but that, combined with app/components/bs-form/element.js, which takes the server-side messages and places them in the customError
property (as a computed property), I get a combination of client- and server-side validation that works. At least for me.