Fix main process exiting while performing finding primary node.

Fix main process exiting while performing finding primary node.

Pgpool-II main process tries to find primary node whenever the cluster
status is changed by failover/failback. While doing it, if a backend
is failing or shutting down, socket write to the backend could
fail. Unfortunately in the case do_query() throws FATAL error, which
makes the Pgpool-II main process die like this.

2022-03-04 18:13:12.711: main pid 795826: WARNING:  write on backend 1 failed with error :"Broken pipe"
2022-03-04 18:13:12.711: main pid 795826: DETAIL:  while trying to write data from offset: 0 wlen: 32
2022-03-04 18:13:12.711: main pid 795826: LOG:  notice_backend_error: called from pgpool main. ignored.
2022-03-04 18:13:12.711: main pid 795826: LOG:  unable to flush data to backend
2022-03-04 18:13:12.711: main pid 795826: DETAIL:  do not failover because I am the main process
2022-03-04 18:13:12.711: main pid 795826: FATAL:  Backend throw an error message
2022-03-04 18:13:12.711: main pid 795826: DETAIL:  Exiting current session because of an error from backend
2022-03-04 18:13:12.711: main pid 795826: HINT:  BACKEND Error: "terminating connection due to administrator command"
2022-03-04 18:13:12.715: main pid 795826: LOG:  shutting down

To prevent it, change ereport(FATAL) to ereport(ERROR) in do_query().