fix flickering image. (#343)

lib/models/app_exception.dart

@@ -0,0 +1,15 @@
+typedef AppExceptionHandler = void Function(AppException);
+
+class AppException implements Exception {
+  AppException({
+    required this.message,
+    this.stackTrace,
+  });
+
+  final String message;
+  final StackTrace? stackTrace;
+}
+
+class RateLimitedException extends AppException {
+  RateLimitedException() : super(message: 'Rate limited...');
+}