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

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
slave_print_error()
- 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*.