Frontend cleanup in "Show rebase pre-receive error to user"
The following discussion from !75496 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.
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