post_object(): return response content in case of an error

journalmarks/static/journalmarks.js

@@ -150,7 +150,7 @@ function post_object(url, data) {
             if (req.status == 200)
                 resolve(JSON.parse(req.response));
             else
-                reject(Error(req.statusText));
+                reject(Error(req.response));
         };
         req.onerror = function() {
             reject(Error("Network Error"));