add some RPC error reporting

functions.js

@@ -307,6 +307,12 @@ if (!xmlhttp_rpc && typeof XMLHttpRequest!='undefined') {
 
 function label_counters_callback() {
 	if (xmlhttp_rpc.readyState == 4) {
+
+		if (!xmlhttp.responseXML) {
+			notify("label_counters_callback: backend did not return valid XML");
+			return;
+		}
+
 		var reply = xmlhttp_rpc.responseXML.firstChild;
 
 		var f_document = parent.frames["feeds-frame"].document;

tt-rss.js

@@ -70,8 +70,18 @@ function refetch_callback() {
 		document.title = "Tiny Tiny RSS";
 		notify("All feeds updated.");
 
+		if (!xmlhttp.responseXML) {
+			notify("refetch_callback: backend did not return valid XML");
+			return;
+		}
+
 		var reply = xmlhttp.responseXML.firstChild;
 
+		if (!reply) {
+			notify("refetch_callback: backend did not return expected XML object");
+			return;
+		}
+
 		var f_document = window.frames["feeds-frame"].document;
 
 		for (var l = 0; l < reply.childNodes.length; l++) {