26.3.2 Deriving Error enums

[mgeisler]

Discussed in #1281

^{Originally posted by njr0 September 29, 2023}
I think a little but more explanation would be helpful here.

I don't think the course has mentioned positional arguments to format strings before, and though I think people will guess that's what "{0}" is, it's not immedately apparent, looking at the code, that there's a argument for it to format.

Similarly, I think a few words about #[error and #[from might be useful.

[fw-immunant]

See #2027.