More messages.
Testcase for bug 651.


client/mysqltest.c:
  More explicit error message if MASTER_POS_WAIT() returns NULL.
mysql-test/r/rpl_loaddata.result:
  result update
mysql-test/r/rpl_master_pos_wait.result:
  result update
mysql-test/t/rpl000001.test:
  sync_with_master (=MASTER_POS_WAIT()) was called when we could expect the SQL slave thread had stopped.
  As I yesterday changed code so that "SQL thread stops => MASTER_POS_WAIT() returns NULL immediately" (bugfix),
  sync_with_master received NULL (on build.mysql.com, not on my machine; this is a question of milliseconds,
  if the slave server will process MASTER_POS_WAIT() before or after the slave SQL thread has stopped), and
  in mysqltest.c, sync_with_master complained that it could not sync.
  So I just remove this sync_with_master, which does not make sense anymore: we just wait for the slave SQL
  thread to stop.
mysql-test/t/rpl_loaddata.test:
  Discovered we had wait_for_slave_to_stop, so used it as it automates things.
mysql-test/t/rpl_master_pos_wait.test:
  Discovered we had 'send' to send a query without waiting for the resultn so could had a testcase for bug 651.
  Shorter timeouts as there is no risk the position is reached.
sql/slave.cc:
  A longer DBUG_PRINT.