When slave stops with error, error message should indicate the failing GTID

When replication stops due to an error, the error message in SHOW SLAVE STATUS
and in the error log really needs to show the GTID (if any) of the transaction
that failed.

This for example is very useful if there is a need to skip that transaction
(by setting @@gtid_slave_pos).

The information is not all that trivial to find currently. One would need to
look into the mysqlbinlog output from the original binlog, and even that is
not foolproof in case of eg. parallel replication.

It would also be very useful to have a column in SHOW SLAVE STATUS that shows
the GTID of any executing transaction.