WL#2668: SHOW SLAVE STATUS: Show errors that keep replication from starting or cause replication to stop

Affects: WorkLog-3.4   —   Status: Un-Assigned

Any error, or warning that keep replication from starting or that causes it to
stop should be listed when the user does mysql> SHOW SLAVE STATUS\G;

Notes from Guilhem Bichot:

If somebody decided that even this Slave_IO_State is not enough, then we should
create a WL task. But if I was asked if this is enough, I'd say yes it is enough.

The WL task could read:
- change some of sql_print_*() in slave.cc, sql_repl.cc to
- modifiy slave_print_error() to mention if it's an error of the I/O or of the
SQL thread, and maybe sometimes we don't want the "Error"
word because it's more of a warning (like "lost connection, trying to
reconnect")?, but then we have the issue that the column in SHOW SLAVE STATUS is
named Last_Slave_*Error*.