Rollup merge of #115278 - RalfJung:removed-error-codes, r=GuillaumeGomez

tell people what to do when removing an error code

Currently tidy and CI send developers on a wild goose chase:
- you edit the code
- CI/tidy tells you that an error code is gone, so you remove it from the list
- CI/tidy tells you that the markdown file is stale, so you remove that as well
- CI (but not tidy) tells you not to remove an error description and copy what E0001 does

Let's be nice to people and directly tell them what to do rather than making them follow misleading breadcrumbs.

r? ``@GuillaumeGomez``

Author: matthiaskrgr