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/rkgk/src/haku.rs

@@ -111,6 +111,8 @@ impl Haku {
         let chunk_id = self.system.add_chunk(chunk).context("too many chunks")?;
         self.brush = Some(chunk_id);
 
+        self.vm.apply_defs(&self.defs);
+
         Ok(())
     }