Adding error logs when unexpected error from gateway

components/device-mgt/org.wso2.carbon.device.mgt.ui/src/main/resources/jaggeryapps/devicemgt/app/modules/business-controllers/device.js

@@ -259,6 +259,10 @@ deviceModule = function () {
             }
             return serviceInvokers.XMLHttp.get(
                 url, function (responsePayload) {
+                    if(!responsePayload["responseText"]){
+                        log.error("Error while fetching device count. API `" + url + "` returns HTTP: " + responsePayload["status"]);
+                        throw constants["ERRORS"]["UNKNOWN_ERROR"];
+                    }
                     return parse(responsePayload["responseText"])["count"];
                 },
                 function (responsePayload) {

components/device-mgt/org.wso2.carbon.device.mgt.ui/src/main/resources/jaggeryapps/devicemgt/app/modules/constants.js

@@ -47,7 +47,8 @@ var LANGUAGE_US = "en_US";
 
 var VENDOR_APPLE = "Apple";
 var ERRORS = {
-    "USER_NOT_FOUND": "USER_NOT_FOUND"
+    "USER_NOT_FOUND": "USER_NOT_FOUND",
+    "UNKNOWN_ERROR": "Unknown Error"
 };
 
 var USER_STORES_NOISY_CHAR = "\"";