message: fix compiler warning
  ```
  error: ignoring return value of function declared with 'warn_unused_result' attribute [-Werror,-Wunused-result]
  ```
diff: @@ -357,10 +357,10 @@ Status S3FileSystem::NewAppendableFile(const string& fname,
  status = reader->Read(offset, kS3ReadAppendableFileBufferSize, &read_chunk,
  buffer.get());
  if (status.ok()) {
  - (*result)->Append(read_chunk);
  + (void)(*result)->Append(read_chunk);
  offset += kS3ReadAppendableFileBufferSize;
  } else if (status.code() == error::OUT_OF_RANGE) {
  - (*result)->Append(read_chunk);
  + (void)(*result)->Append(read_chunk);
  break;
  } else {
  (*result).reset();
  