running slave, just give a warning. Same thing for STOP SLAVE.
This will make scripts writing easier for users.


include/mysqld_error.h:
  New error codes (warning codes in fact) to warn that the slave was already started or stopped
mysql-test/include/master-slave.inc:
  STOP SLAVE now prints a warning, not an error, so we should expect a warning here
sql/share/czech/errmsg.txt:
  New messages to warn that the slave was already started or stopped
sql/share/danish/errmsg.txt:
  New messages to warn that the slave was already started or stopped
sql/share/dutch/errmsg.txt:
  New messages to warn that the slave was already started or stopped
sql/share/english/errmsg.txt:
  New messages to warn that the slave was already started or stopped
sql/share/estonian/errmsg.txt:
  New messages to warn that the slave was already started or stopped
sql/share/french/errmsg.txt:
  New messages to warn that the slave was already started or stopped
sql/share/german/errmsg.txt:
  New messages to warn that the slave was already started or stopped
sql/share/greek/errmsg.txt:
  New messages to warn that the slave was already started or stopped
sql/share/hungarian/errmsg.txt:
  New messages to warn that the slave was already started or stopped
sql/share/italian/errmsg.txt:
  New messages to warn that the slave was already started or stopped
sql/share/japanese/errmsg.txt:
  New messages to warn that the slave was already started or stopped
sql/share/korean/errmsg.txt:
  New messages to warn that the slave was already started or stopped
sql/share/norwegian-ny/errmsg.txt:
  New messages to warn that the slave was already started or stopped
sql/share/norwegian/errmsg.txt:
  New messages to warn that the slave was already started or stopped
sql/share/polish/errmsg.txt:
  New messages to warn that the slave was already started or stopped
sql/share/portuguese/errmsg.txt:
  New messages to warn that the slave was already started or stopped
sql/share/romanian/errmsg.txt:
  New messages to warn that the slave was already started or stopped
sql/share/russian/errmsg.txt:
  New messages to warn that the slave was already started or stopped
sql/share/serbian/errmsg.txt:
  New messages to warn that the slave was already started or stopped
sql/share/slovak/errmsg.txt:
  New messages to warn that the slave was already started or stopped
sql/share/spanish/errmsg.txt:
  New messages to warn that the slave was already started or stopped
sql/share/swedish/errmsg.txt:
  New messages to warn that the slave was already started or stopped
sql/share/ukrainian/errmsg.txt:
  New messages to warn that the slave was already started or stopped
sql/sql_repl.cc:
  Do not give an error when the user does START SLAVE on an already
  running slave, just give a warning. Same thing for STOP SLAVE.