@Override
      public void onFileCopyFailure(Exception e) {
        successFailCount.put("fail", successFailCount.get("fail") + 1);
      }
Tests when fileCopyHandler is failed and onFileCopyFailure is called.
and onFileCopySuccess is not called
ambry-file-transfer/src/test/java/com/github/ambry/filetransfer/FileCopyThreadTest.java