my_error() was invoked in reset_slave()'s with purge_relay_logs()-failing branch
without passing sql_errno to it.

Fixed with setting sql_errno= ER_RELAY_LOG_FAIL in the purge_relay_logs()-failing branch.




sql/sql_repl.cc:
  set sql_errno= ER_RELAY_LOG_FAIL when purge_relay_logs() fails.