[MR Widget V2]: Handle errors in the wrapper component

Now that !93017 (merged) is merged, we need to add the error handling functionality to the component.

The implementation needs to look like the following:

Implementation plan

• https://gitlab.com/gitlab-org/gitlab/-/blob/master/app/assets/javascripts/vue_merge_request_widget/components/widget/widget.vue is where the error logic will be implemented. When there is an error, display the errorText and the error icon.