message: [vm] add a comment about running the failure epilogue
  Closes:
diff: @@ -89,6 +89,12 @@ impl LibraVM {
  let mut session = self.0.new_session(remote_cache);
  match TransactionStatus::from(error_code.clone()) {
  TransactionStatus::Keep(status) => {
  + // The transaction should be charged for gas, so run the epilogue to do that.
  + // This is running in a new session that drops any side effects from the
  + // attempted transaction (e.g., spending funds that were needed to pay for gas),
  + // so even if the previous failure occurred while running the epilogue, it
  + // should not fail now. If it somehow fails here, there is no choice but to
  + // discard the transaction.
  if let Err(e) = self.0.run_failure_epilogue(
  &mut session,
  &mut cost_strategy,
  