How can I disabled a global ember error handler?

Someone (not me) has implemented a global ember error handler as a mixin that is used on all controllers:

export default Ember.Mixin.create({
    handleErrors: function (error) {
        alert(error);
    }
});

import ErrorHandler from '../mixins/error-handler';

export default BaseController.extend(ErrorHandler , {
  ....
});

I'm making an ajax call and under certain circumstances I want it to fail (record is locked in the backend and user shouldn't be able to continue).

I'm return a HTTP error code to my ajax call:

return Ember.$.ajax({
        url:url,
        method:'GET',
        cache:false,
        xhrFields: {
            withCredentials: true
         }
    })
    .fail(( jqXHR) => {
        if (jqXHR.status === 409){
            alert('This record is locked and you cannot create a New Revision until the lock is released by the person currently editing it.');
        }

    })
    .then((response) => {
        this.get('store').pushPayload(response);
        return response;
    });

I want to handle this state here and only here, I want to prevent the error bubbling up to the handleErrors call. I can't figure out how to prevent handleErrors from getting called though.

How can I get this to resolve to an unerror state?

Due to complicated (and overly complicated design not done by myself...) reasons I can't remove the mixin and most of the time this is doing an ok job.