fix updated defs not being applied properly to VM in frontend and backend

this fixes the case where

	(def botsbuildbots (fn () (botsbuildbots))) (botsbuildbots)

would not run properly (return with a "set def index out of bounds" error)

also make exceptions store String instead of &'static str for better error reporting

closes #33

crates/haku/src/render.rs

@@ -43,8 +43,8 @@ impl<'a> Renderer<'a> {
         }
     }
 
-    fn create_exception(_vm: &Vm, _at: Value, message: &'static str) -> Exception {
-        Exception { message }
+    fn create_exception(vm: &Vm, _at: Value, message: &'static str) -> Exception {
+        vm.create_exception(message)
     }
 
     fn transform(&self) -> Transform {