Currently, rpl_semi_sync is failing in PB due to the warning message:

  "Slave SQL: slave SQL thread is being stopped in the middle of "
  "applying of a group having updated a non-transaction table; "
  "waiting for the group completion ..."

The problem started happening after the fix for BUG#11762407 what was
automatically suppressing some warning messages.

To fix the current issue, we suppress the aforementioned warning message
and exploit the opportunity to make the sentence clearer.