Fix watchdog to print inappropriate NOTICE message.

Fix watchdog to print inappropriate NOTICE message.

read_ipc_socket_and_process() printed a notice message every time when
it wrote commands to IPC socket even if it was successful. Fix this to
print the notice message only when the write failed.

The reason why this bug was not recognized is, the message appears
only when log_min_messages is set to notice or higher.

Discussion: https://github.com/pgpool/pgpool2/issues/121
Backpatch-through: v4.2