"503 Maintenance" error handling

refs https://github.com/TryGhost/Ghost/issues/6976
- adds custom `MaintenanceError` and associated error checking functions
- updates app route and notifications service to handle `503` errors via the `upgrade-status` service

ghost/admin/app/mirage/config.js

@@ -9,6 +9,14 @@ const {
 } = Ember;
 
 /* jshint unused:false */
+function maintenanceResponse() {
+    return new Mirage.Response(503, {}, {
+        errors: [{
+            errorType: 'Maintenance'
+        }]
+    });
+}
+
 function versionMismatchResponse() {
     return new Mirage.Response(400, {}, {
         errors: [{

ghost/admin/app/routes/application.js

@@ -176,6 +176,14 @@ export default Route.extend(ApplicationRouteMixin, ShortcutsRoute, {
                         this.get('upgradeStatus').requireUpgrade();
                         return false;
 
+                    case 'Maintenance':
+                        if (transition) {
+                            transition.abort();
+                        }
+
+                        this.get('upgradeStatus').maintenanceAlert();
+                        return false;
+
                     default:
                         this.get('notifications').showAPIError(error);
                         // don't show the 500 page if we weren't navigating

ghost/admin/app/services/ajax.js

@@ -70,6 +70,24 @@ export function isUnsupportedMediaTypeError(errorOrStatus) {
     }
 }
 
+/* Maintenance error */
+
+export function MaintenanceError(errors) {
+    AjaxError.call(this, errors, 'Ghost is currently undergoing maintenance, please wait a moment then retry.');
+}
+
+MaintenanceError.prototype = Object.create(AjaxError.prototype);
+
+export function isMaintenanceError(errorOrStatus) {
+    if (isAjaxError(errorOrStatus)) {
+        return errorOrStatus instanceof MaintenanceError;
+    } else if (errorOrStatus && get(errorOrStatus, 'isAdapterError')) {
+        return get(errorOrStatus, 'errors.firstObject.errorType') === 'Maintenance';
+    } else {
+        return errorOrStatus === 503;
+    }
+}
+
 /* end: custom error types */
 
 export default AjaxService.extend({
@@ -99,6 +117,8 @@ export default AjaxService.extend({
             return new RequestEntityTooLargeError(payload.errors);
         } else if (this.isUnsupportedMediaTypeError(status, headers, payload)) {
             return new UnsupportedMediaTypeError(payload.errors);
+        } else if (this.isMaintenanceError(status, headers, payload)) {
+            return new MaintenanceError(payload.errors);
         }
 
         return this._super(...arguments);
@@ -136,5 +156,9 @@ export default AjaxService.extend({
 
     isUnsupportedMediaTypeError(status/*, headers, payload */) {
         return isUnsupportedMediaTypeError(status);
+    },
+
+    isMaintenanceError(status, headers, payload) {
+        return isMaintenanceError(status, payload);
     }
 });

ghost/admin/app/services/notifications.js

@@ -4,9 +4,12 @@ import {A as emberA, isEmberArray} from 'ember-array/utils';
 import get from 'ember-metal/get';
 import set from 'ember-metal/set';
 import injectService from 'ember-service/inject';
-import {isVersionMismatchError} from 'ghost-admin/services/ajax';
 import {isBlank} from 'ember-utils';
 import {dasherize} from 'ember-string';
+import {
+    isMaintenanceError,
+    isVersionMismatchError
+} from 'ghost-admin/services/ajax';
 
 // Notification keys take the form of "noun.verb.message", eg:
 //
@@ -88,6 +91,9 @@ export default Service.extend({
         // handle "global" errors
         if (isVersionMismatchError(resp)) {
             return this.get('upgradeStatus').requireUpgrade();
+        } else if (isMaintenanceError(resp)) {
+            console.log(this.get('upgradeStatus'));
+            return this.get('upgradeStatus').maintenanceAlert();
         }
 
         // loop over Ember Data / ember-ajax errors object

ghost/admin/app/services/upgrade-status.js

@@ -6,6 +6,13 @@ export default Service.extend({
 
     notifications: injectService(),
 
+    maintenanceAlert() {
+        this.get('notifications').showAlert(
+            'Sorry, Ghost is currently undergoing maintenance, please wait a moment then try again.',
+            {type: 'error', key: 'api-error.under-maintenance'}
+        );
+    },
+
     requireUpgrade() {
         this.set('isRequired', true);
         this.get('notifications').showAlert(