fix(io): rethrow exceptions from S3 delete helpers

wait() leaves an exception stored in a helper's future, so a delete that
threw was neither counted nor reported, and DeleteFiles could return
success. get() rethrows it on the calling thread, as the sequential loop
did.

Author: plusplusjiajia

src/iceberg/arrow/s3/arrow_s3_file_io.cc

@@ -318,7 +318,7 @@ Status ArrowS3FileIO::DeleteFiles(const std::vector<std::string>& file_locations
   }
   delete_remaining();
   for (auto& helper : helpers) {
-    helper.wait();
+    helper.get();  // Rethrows what a helper threw.
   }
   if (failed > 0) {
     return IOError("Failed to delete {} of {} files", failed.load(),