How do you get a display an SignInFailure error message in Firebase-ui web?

Firstly, using firebaseui-angular package to implement FirebaseUI.

I've set up anonymous guest accounts and I'm working on upgrading these accounts to real accounts. In my application, I do not want to merge accounts - if the user tries to upgrade an anonymous account to one that already exists I want to display an error message.

This is my code

login.component.html

<firebase-ui (signInSuccessWithAuthResult)="onSuccess($event)" (signInFailure)="onFailure($event)"></firebase-ui>

login.component.ts

  onFailure(event) {
    console.log(event);
    if (event.code == 'firebaseui/anonymous-upgrade-merge-conflict') {
      // what do I put here?
    }
  }

I can confirm that if they are an anonymous user and they try to upgrade with an account that already exists they get a signInFailure event with anonymous-upgrade-merge-conflict.

This is consistent with the documentation (https://firebase.google.com/docs/auth/web/firebaseui#handling_anonymous_user_upgrade_merge_conflicts)

My problem is I don't want to continue merging the account, I want to display an error message telling them they can't merge.

How do I display a custom error message on SignInFailure? Does the package support this? Do I need to display my own error message outside of FirebaseUI?