Mention std::error::Error in speaker notes, and other useful traits.

src/error-handling/converting-error-types.md

@@ -57,4 +57,9 @@ Key points:
 * The `username` variable can be either `Ok(string)` or `Err(error)`.
 * Use the `fs::write` call to test out the different scenarios: no file, empty file, file with username.
 
+It is good practice for all error types to implement `std::error::Error`, which requires `Debug` and
+`Display`. It's generally helpful for them to implement `Clone` and `Eq` too where possible, to make
+life easier for tests and consumers of your library. In this case we can't easily do so, because
+`io::Error` doesn't implement them.
+
 </details>

src/error-handling/deriving-error-enums.md

@@ -8,7 +8,7 @@ use std::{fs, io};
 use std::io::Read;
 use thiserror::Error;
 
-#[derive(Error, Debug)]
+#[derive(Debug, Error)]
 enum ReadUsernameError {
     #[error("Could not read: {0}")]
     IoError(#[from] io::Error),
@@ -33,3 +33,13 @@ fn main() {
     }
 }
 ```
+
+<details>
+
+`thiserror`'s derive macro automatically implements `std::error::Error`, and optionally `Display`
+(if the `#[error(...)]` attributes are provided) and `From` (if the `#[from]` attribute is added).
+It also works for structs.
+
+It doesn't affect your public API, which makes it good for libraries.
+
+</details>