TF-3189 new option to copy a folder's subaddress

lib/features/mailbox/domain/exceptions/empty_folder_name_exception.dart

@@ -0,0 +1,9 @@
+
+class EmptyFolderNameException implements Exception {
+  final String folderName;
+
+  EmptyFolderNameException(this.folderName);
+
+  @override
+  String toString() => 'EmptyFolderNameException: Folder name should not be empty: $folderName';
+}

lib/features/mailbox/domain/exceptions/invalid_mail_format_exception.dart

@@ -0,0 +1,9 @@
+
+class InvalidMailFormatException implements Exception {
+  final String mail;
+
+  InvalidMailFormatException(this.mail);
+
+  @override
+  String toString() => 'InvalidMailFormatException: $mail';
+}