also return evaluation errors

src/lib.rs

@@ -74,7 +74,11 @@ pub fn eval_boolean(s: &str, js_trace: JsValue) -> JsValue {
     let parse_result = argus::parse_str(s);
     let expr = match parse_result {
         Ok(expr) => expr,
-        Err(e) => return JsValue::from_str("failed to parse expression"),
+        Err(e) => {
+            let messages: Vec<String> = e.iter().map(|e| e.to_string()).collect();
+            let joined = messages.join(",");
+            return JsValue::from_str(format!("failed to parse expression: {}", joined).as_str());
+        },
     };
     // evaluate property on trace
     let eval_result = match expr {