When you make a query into Solr via Solr Admin Page and if error occurs there writes "Loading.." and does nothing.
i.e. if you write an invalid Request Handler at Query page even response is 404 Not Found "Loading..." is still there. I've opened a Jira for it: https://issues.apache.org/jira/browse/SOLR-5419 and applied a patch. I've also fixed logic at Ajax request. -- Thanks; Furkan KAMACI
