return JSON error to see what went wrong

src/lib.rs

@@ -52,7 +52,7 @@ pub fn eval_boolean(s: &str, js_trace: JsValue) -> JsValue {
     // convert input (a sequence of state-objects) to a Trace that argus accepts:
     let trace: StateBuddyTrace = match from_value(js_trace) {
         Ok(trace) => trace,
-        Err(e) => return JsValue::from_str("failed to parse JSON"),
+        Err(e) => return JsValue::from_str(("failed to parse JSON: ".to_owned() + &e.to_string()).as_str()),
     };
     let mut traceMap = HashMap::<&str, Signal<f64>>::new();
     for entry in &trace.entries {