reorganize module

_archive/rhai_engine/rhaibook/language/throw.md

@@ -0,0 +1,56 @@
+Throw Exception on Error
+========================
+
+{{#include ../links.md}}
+
+All [`Engine`] evaluation API methods return `Result<T, Box<rhai::EvalAltResult>>`
+with `EvalAltResult` holding error information.
+
+To deliberately return an error, use the `throw` keyword.
+
+```js
+if some_bad_condition_has_happened {
+    throw error;    // 'throw' any value as the exception
+}
+
+throw;              // defaults to '()'
+```
+
+Exceptions thrown via `throw` in the script can be captured in Rust by matching
+`EvalAltResult::ErrorRuntime(value, position)` with the exception value captured by `value`.
+
+```rust
+let result = engine.eval::<i64>(
+"
+    let x = 42;
+
+    if x > 0 {
+        throw x;
+    }
+").expect_err();
+
+println!("{result}");       // prints "Runtime error: 42 (line 5, position 15)"
+```
+
+
+Catch a Thrown Exception
+------------------------
+
+It is possible to _catch_ an exception instead of having it abort the evaluation
+of the entire script via the [`try` ... `catch`][`try`]
+statement common to many C-like languages.
+
+```js
+fn code_that_throws() {
+    throw 42;
+}
+
+try
+{
+    code_that_throws();
+}
+catch (err)         // 'err' captures the thrown exception value
+{
+    print(err);     // prints 42
+}
+```