create backup only when receiver found

include/TransferState.hpp

@@ -11,14 +11,16 @@ struct TransferState {
     std::atomic<uint64_t> bytes_total{0};
 
     std::string    status;
+    std::string    fail_reason;
     mutable std::mutex status_mutex;
 
     void reset() {
-        done             = false;
-        cancelled        = false;
+        done              = false;
+        cancelled         = false;
         connection_failed = false;
-        bytes_done       = 0;
-        bytes_total      = 0;
+        bytes_done        = 0;
+        bytes_total       = 0;
+        fail_reason.clear();
         std::lock_guard<std::mutex> lock(status_mutex);
         status.clear();
     }