// in model.Traits:

const ErrCodeTraitUpdateOverridable = 403

...

if !backfilledSinceUpdate {
        nbErr := TraitNotYetBackfilledError("the trait hasn't been backfilled, and thus can't be updated yet. This can be overridden.")
        return nbErr
}

if strings.Count(search.Search, "tag:trait") > 1 {
    return SearchHasMultipleTraitReferencesError("the search contains multiple trait references",
            "search_body", search.Search,
            "trait_tag_name", tagName)
}

// in controller:

gErr := c.Traits.ValidateUpdate(trait, search)
if gErr != nil {
    switch svcErr := gErr.(type) {
        case goa.ErrorResponse:
            if svcErr.Status == model.ErrCodeTraitUpdateOverridable {
                    if *ctx.Force {
                        fbk = fmt.Sprintf("ERR_OVERRIDE:\n%s\n", gErr.Error())
                    }
            }
            break
    default:
        break
    }
return ctx.BadRequest(gErr)
}