doc: book section on error handling (#393)

Author: molpopgen

book/src/SUMMARY.md

@@ -19,6 +19,8 @@
   - [Defining metadata types in rust](./metadata_derive.md)
   - [Metadata and tables](./metadata_tables.md)
   - [Metadata schema](./metadata_schema.md)
+
+* [Error handling](./error_handling.md)
 
 
 [Crate prelude](./prelude.md)

book/src/error_handling.md

@@ -0,0 +1,17 @@
+# Error handling <img align="right" width="73" height="45" src="https://raw.githubusercontent.com/tskit-dev/administrative/main/logos/svg/tskit-rust/Tskit_rust_logo.eps.svg">
+
+The error type is [`tskit::error::TskitError`](https://docs.rs/tskit/latest/tskit/error/enum.TskitError.html#).
+
+This type implements `Display` and may thus be printed via:
+
+```rust, noplayground, ignore
+let x = match operation() {
+    Ok(y) => y,
+    Err(e) => panic("{}", e);
+};
+```
+
+The enum variant `TskitError::ErrorCode` represents integer return values from `tskit-c`.
+When printed via `Display`, the detailed error message is fetched.
+When printed via `Debug`, the numerical error code is shown.
+