TF-233 Add mailbox name validator

lib/features/mailbox_creator/domain/state/verify_name_view_state.dart

@@ -0,0 +1,17 @@
+
+import 'package:core/core.dart';
+
+class VerifyNameViewState extends UIState {
+  VerifyNameViewState();
+
+  @override
+  List<Object> get props => [];
+}
+
+class VerifyNameFailure extends FeatureFailure {
+  final exception;
+
+  VerifyNameFailure(this.exception);
+  @override
+  List<Object> get props => [exception];
+}