Frontend cleanup in "Show rebase pre-receive error to user"

The following discussion from !75496 (merged) should be addressed:

It seems like we have extraneous error messages popping up below: one flash message at the top, a flash message in the widget, and a rebasingError below the widget.

We seem to create a flash message in https://gitlab.com/gitlab-org/gitlab/blob/809b12710d0e65671cddfc8a6c8a88fe5d48c30f/app/assets/javascripts/vue_merge_request_widget/components/states/mr_widget_rebase.vue#L124-126 even though we also display rebasingError in https://gitlab.com/gitlab-org/gitlab/blob/809b12710d0e65671cddfc8a6c8a88fe5d48c30f/app/assets/javascripts/vue_merge_request_widget/components/states/mr_widget_rebase.vue#L190. Maybe we can drop rebasingError since it seems redundant?

I'm also inclined to drop the top flash message when merge_error is present.

Initial failure

After reload