Rollup merge of #106341 - Ezrashaw:refactor-error-code-tidy-check, r=mejrs,klensy...

Rollup merge of #106341 - Ezrashaw:refactor-error-code-tidy-check, r=mejrs,klensy,GuillaumeGomez

refactor: clean up `errors.rs` and `error_codes_check.rs`

`errors.rs` is basically unused now, `error_codes_check.rs` is useful but not well commented, etc. It also doesn't check certain things which are certainly not correct. For example, `E0505` has a UI test in `src/test/ui/error-codes/` but that test actually outputs `E0504`?! Other issues like these exist. I've implemented these with "warnings" which are a bit rough around the edges but should be removed eventually.

r? `@GuillaumeGomez` (again not sure if you want to review but its relevant to you)