Handle AddOnEligibleUserList component Apollo fetch errors in-component

What does this MR do and why?

Resolves the issue in https://gitlab.com/gitlab-org/gitlab/-/issues/469221+ where the subscription permissions GraphQL fetch errors are not displayed in the view. This is because the AddOnEligibleUserList component passes the fetch errors to its parent, which does not handle that event. This MR lets the component handle the error through its error alert component. It also removes the forwardException method which emits the error up to the parent, since that is no longer needed or used.

References

Related issue: https://gitlab.com/gitlab-org/gitlab/-/issues/469221+

Screenshots or screen recordings

Before	After

How to set up and validate locally

1. Start GDK, CDot, and Zsim.
2. Create a new group with at least 3 users and purchase an Ultimate subscription with 2 Duo Enterprise seats.
3. Force a GraphQL endpoint in the getSubscriptionPermissionsData query, by renaming one of the query arguments to a non-existent variable, for example.
4. Navigate to /groups/:group_name/-/settings/gitlab_duo/seat_utilization and confirm an error alert is present, as per the screenshots above.

MR acceptance checklist

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