Improve error message when runner deletion fails
Description
Implement error message appearance as in (using flash or gl-alert):
![#339653[Delete-fail.png]](https://gitlab.com/gitlab-org/gitlab/-/issues/339653/designs/Delete-fail.png){kind=link}
Original MR note:
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@mrincon started a discussion: @gdoyle I haven't completed the full scope of #339653[Delete-fail.png]
So I'll open a separate follow-up.
Edited by Miguel Rincon