Handle errors thrown during app boot

closes https://github.com/TryGhost/Ghost/issues/9394
- add `application-error.hbs` template so that we show an error screen for errors during app boot
- track the application route loading state so that we can fall back to the default error handling for errors that usually only show an alert

ghost/admin/app/routes/application.js

@@ -82,8 +82,13 @@ export default Route.extend(ApplicationRouteMixin, ShortcutsRoute, {
                 settingsPromise,
                 privateConfigPromise,
                 tourPromise
-            ]);
+            ]).then((results) => {
+                this._appLoaded = true;
+                return results;
+            });
         }
+
+        this._appLoaded = true;
     },
 
     actions: {
@@ -180,7 +185,10 @@ export default Route.extend(ApplicationRouteMixin, ShortcutsRoute, {
                 }
 
                 this.get('upgradeStatus').requireUpgrade();
-                return false;
+
+                if (this._appLoaded) {
+                    return false;
+                }
             }
 
             if (isMaintenanceError(error)) {
@@ -189,7 +197,10 @@ export default Route.extend(ApplicationRouteMixin, ShortcutsRoute, {
                 }
 
                 this.get('upgradeStatus').maintenanceAlert();
-                return false;
+
+                if (this._appLoaded) {
+                    return false;
+                }
             }
 
             if (isAjaxError(error) || error && error.payload && isEmberArray(error.payload.errors)) {

ghost/admin/app/templates/application-error.hbs

@@ -0,0 +1,15 @@
+<div class="gh-view">
+    <section class="error-content error-404 js-error-container">
+        <section class="error-details">
+            <img class="error-ghost" src="assets/img/404-ghost@2x.png" srcset="assets/img/404-ghost.png 1x, assets/img/404-ghost@2x.png 2x" />
+            <section class="error-message">
+                <h1 class="error-code">{{model.code}}</h1>
+                <h2 class="error-description">
+                    {{or model.payload.errors.firstObject.message model.message}}
+                </h2>
+            </section>
+        </section>
+    </section>
+</div>
+
+{{ember-load-remover}}