better error message and loading view

lib/api/api.dart

@@ -2,7 +2,11 @@ import 'dart:async';
 import 'dart:convert';
 
 import 'package:http/http.dart' as http;
-import 'package:openmediacentermobile/api/token.dart';
+import 'token.dart';
+
+class TokenException implements Exception {
+  String wdExpMsg() => 'Invalid token in storage!';
+}
 
 class API {
   static Future<String> query(
@@ -20,7 +24,7 @@ class API {
 
       return resp.body;
     } else {
-      return "";
+      throw TokenException();
     }
   }
 }

lib/preview/preview_grid.dart

@@ -56,7 +56,16 @@ class _PreviewGridState extends State<PreviewGrid> {
           builder:
               (BuildContext context, AsyncSnapshot<List<VideoT>> snapshot) {
             if (snapshot.hasError) {
-              return Text("Error");
+              return Column(
+                children: [
+                  Text("Error"),
+                  TextButton(
+                      onPressed: () {
+                        loadData();
+                      },
+                      child: Text("Reload page"))
+                ],
+              );
             } else if (snapshot.hasData) {
               return _mainGrid(snapshot.data!, width);
             } else {

lib/screen_loading.dart

@@ -5,10 +5,11 @@ class ScreenLoading extends StatelessWidget {
 
   @override
   Widget build(BuildContext context) {
-    return Column(
-        mainAxisAlignment: MainAxisAlignment.center,
-        crossAxisAlignment: CrossAxisAlignment.center,
-        children: const <Widget>[
+    return Center(
+        child: Column(
+            mainAxisAlignment: MainAxisAlignment.center,
+            crossAxisAlignment: CrossAxisAlignment.center,
+            children: const <Widget>[
           SizedBox(
             width: 60,
             height: 60,
@@ -18,6 +19,6 @@ class ScreenLoading extends StatelessWidget {
             padding: EdgeInsets.only(top: 16),
             child: Text('Awaiting result...'),
           )
-        ]);
+        ]));
   }
 }