Use ? operator for examples.

examples/simple_fn.rs

@@ -1,6 +1,6 @@
-use rhai::{Engine, RegisterFn};
+use rhai::{Engine, EvalAltResult, RegisterFn};
 
-fn main() {
+fn main() -> Result<(), EvalAltResult> {
     let mut engine = Engine::new();
 
     fn add(x: i64, y: i64) -> i64 {
@@ -9,7 +9,9 @@ fn main() {
 
     engine.register_fn("add", add);
 
-    if let Ok(result) = engine.eval::<i64>("add(40, 2)") {
-        println!("Answer: {}", result); // prints 42
-    }
+    let result = engine.eval::<i64>("add(40, 2)")?;
+
+    println!("Answer: {}", result); // prints 42
+
+    Ok(())
 }