Custom error enums

Small Rust programs often use an enum for domain errors. This theme teaches error variants, carrying context, Display, and using From so ? can convert lower-level errors. Planned exercises: 1. choose an error enum. 2. recognise context-carrying variants. 3. choose From for conversion. 4. fill an Err(MyError::...). 5. fill a Display arm. 6. write a domain error enum. 7. write a conversion with From. 8. translate TS custom error classes. 9. fix an error propagation type mismatch.