Fix bug

lib/flutter_openim_sdk.dart

@@ -9,6 +9,7 @@ export 'src/enum/im_platform.dart';
 export 'src/enum/listener_type.dart';
 export 'src/enum/message_status.dart';
 export 'src/enum/message_type.dart';
+export 'src/enum/sdk_error_code.dart';
 export 'src/listener/advanced_msg_listener.dart';
 export 'src/listener/connect_listener.dart';
 export 'src/listener/conversation_listener.dart';

lib/src/enum/sdk_error_code.dart

@@ -0,0 +1,14 @@
+/// 消息发送失败
+class MessageFailedCode {
+  /// 被对方拉黑
+  static const int blockedByFriend = 600;
+
+  /// 被对方删除
+  static const int deletedByFriend = 601;
+}
+
+/// 添加好友失败
+class AddFriendFailedCode {
+  /// 该用户已设置不可添加
+  static const int refuseToAddFriends = 10007;
+}