message: Improve the error message when failing to write events.
  The current error message looks like:
  "Failed to sync 10 to <path>"
diff: @@ -122,9 +122,11 @@ Status EventsWriter::Flush() {
  CHECK(recordio_file_ != nullptr) << "Unexpected NULL file";
  TF_RETURN_WITH_CONTEXT_IF_ERROR(recordio_writer_->Flush(), "Failed to flush ",
  - num_outstanding_events_, " to ", filename_);
  + num_outstanding_events_, " events to ",
  + filename_);
  TF_RETURN_WITH_CONTEXT_IF_ERROR(recordio_file_->Sync(), "Failed to sync ",
  - num_outstanding_events_, " to ", filename_);
  + num_outstanding_events_, " events to ",
  + filename_);
  // The FileStillExists() condition is necessary because
  // recordio_writer_->Sync() can return OK even if the underlying
  @@ -135,7 +137,8 @@ Status EventsWriter::Flush() {
  // disappearing file, in case for some file system File::Exists() is
  // false after File::Open() but before File::Sync().
  TF_RETURN_WITH_CONTEXT_IF_ERROR(FileStillExists(), "Failed to flush ",
  - num_outstanding_events_, " to ", filename_);
  + num_outstanding_events_, " events to ",
  + filename_);
  VLOG(1) << "Wrote " << num_outstanding_events_ << " events to disk.";
  num_outstanding_events_ = 0;
  return Status::OK();
  