Update tidy check for error codes testing

[GuillaumeGomez]

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).

[GuillaumeGomez]

Updated.

[Mark-Simulacrum]

Could you also say a bit about what this is doing? i.e., what is the goal of this PR, what are we trying to change, and why? I don't have the time to dig into that myself so it'd make an r+ easier on my end.

[GuillaumeGomez]

Sure. 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).

[Mark-Simulacrum]

Could we use fs::read_to_string here? Seems like there's no reason why not?

[Mark-Simulacrum]

For the future it'd be great to include that information in the commit; I've copied it into the PR description.

r=me with last nit fixed

[GuillaumeGomez]

Indeed, sorry...

[Mark-Simulacrum]

@bors r+

[bors]

📌 Commit df9fed1 has been approved by Mark-Simulacrum

[bors]

⌛ Testing commit df9fed1 with merge d902539...