Rollup merge of #66574 - GuillaumeGomez:update-tidy-err-code-check, r=Mark-Simulacrum
Rollup merge of #66574 - GuillaumeGomez:update-tidy-err-code-check, r=Mark-Simulacrum

Update tidy check for error codes testing

Now that all the error codes have been grouped into one file, we moved the long error explanations into markdown files, which means that we need to read those markdown file to also count their code blocks as well.

So 2 "big" things happened here:
* No more need to keep the current error code when reading the `error_codes.rs` file since it's been put into the equivalent markdown file.
* Need to instead read the markdown file (but it's simpler since I can just look for code blocks directly).

aidancully/rust,graydon/rust,aidancully/rust,graydon/rust,graydon/rust,graydon/rust,aidancully/rust,aidancully/rust,aidancully/rust,aidancully/rust,graydon/rust,graydon/rust