cvm-rs: Actually throw Error on guac decode errors

we were throwing String or something before..

cvm-rs/src/guac_js.rs

@@ -21,8 +21,7 @@ fn guac_decode_impl<'a>(cx: &mut FunctionContext<'a>) -> JsResult<'a, JsArray> {
         }
 
         Err(e) => {
-            let err = cx.string(format!("{}", e));
+            return cx.throw_error(format!("{}", e));
-            return cx.throw(err);
         }
     }
 }