purge_relay_logs() did not propagate an error happend in count_relay_log_space().

Fixed with the suggestesd setting the error= true.
Note, propagation generally out of purge_relay_logs() was fixed for Bug #44179,
and the issue does not exist in 6.0 thanks to a patch for WL#2775.



sql/rpl_rli.cc:
  the error is set on if count_relay_log_space(rli) fails.