Before this patch, failures to write to the log tables (mysql.slow_log
and mysql.general_log) were improperly printed (the time was printed twice),
or not printed at all.

With this patch, failures to write to the log tables is reported in the
error log, for all cases of failures.


mysql-test/r/log_tables.result:
  Bug#27858 (Failing to log to a log table doesn't log anything to error log)
mysql-test/t/log_tables.test:
  Bug#27858 (Failing to log to a log table doesn't log anything to error log)
sql/log.cc:
  Bug#27858 (Failing to log to a log table doesn't log anything to error log)