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();
     }
   }
 }