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-wasm/src/lib.rs

@@ -328,6 +328,8 @@ unsafe extern "C" fn haku_compile_brush(
     };
     brush.state = BrushState::Ready(chunk_id);
 
+    instance.vm.apply_defs(&instance.defs);
+
     info!("brush compiled into {chunk_id:?}");
 
     StatusCode::Ok