Suppress unnecessary error message when there's no standby server.

Suppress unnecessary error message when there's no standby server.

Even of there's no standby server anymore, worker process keeps on
checking result row from pg_stat_replication(). If
pg_stat_replication() returns no row, the worker process complained it
every sr_check_period and the log was flooded by them. To prevent this
get_query_result() is changed to return -2 if there's no row in the
result. Also the error message in get_query_result() is downgraded
from LOG to DEBUG1. If -2 is returned and number of active standby is
less than 1 the caller does not complain any more.