Add Rhai book.

doc/src/language/throw.md

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+Throw Exception on Error
+=======================
+
+{{#include ../links.md}}
+
+All of [`Engine`]'s evaluation/consuming methods return `Result<T, Box<rhai::EvalAltResult>>`
+with `EvalAltResult` holding error information.
+
+To deliberately return an error during an evaluation, use the `throw` keyword.
+
+```rust
+if some_bad_condition_has_happened {
+    throw error;    // 'throw' takes a string as the exception text
+}
+
+throw;              // defaults to empty exception text: ""
+```
+
+Exceptions thrown via `throw` in the script can be captured by matching `Err(Box<EvalAltResult::ErrorRuntime(` _reason_ `,` _position_ `)>)`
+with the exception text captured by the first parameter.
+
+```rust
+let result = engine.eval::<i64>(r#"
+    let x = 42;
+
+    if x > 0 {
+        throw x + " is too large!";
+    }
+"#);
+
+println!(result);   // prints "Runtime error: 42 is too large! (line 5, position 15)"
+```