Error code linter should detect when an error code isn't checked in its explanation

From this [comment](https://github.com/rust-lang/rust/pull/83213/files/a4d729a6d8d4e5c003de65b7599e6f1f75bdb7d2#diff-3080a67cb86325ee79bb8a820e1f4f533616730880ad3dd741219638cd0a0744).