When we are writing a transaction to the binlog, we log BEGIN/COMMIT with zero error code.
Example: all statements of trans succeeded, connection lost and so implicit rollback:
we don't want ER_NET* errors to be logged in the BEGIN/ROLLBACK events, while statement
events have 0. If there was really a serious error code, it's already in the statement events.


sql/log.cc:
  When we write the cached binlog segment to disk binlog at COMMIT/ROLLBACK time:
  imagine this is rollback due to net timeout, after all statements of
  the transaction succeeded. Then we want a zero-error code in BEGIN.
  In other words, if there was a really serious error code it's already
  in the transaction's statement events.
sql/sql_table.cc:
  out of date comment