Fix the return code when stop the node that is started by pg_autoctl (#572)

When the node is not started by pg_autoctl, the pid file doesn't
exist, so pg_autoctl raises an error(calls log_fatal(...)). But
log_fatal() is not a no-return function, it finally returns 0 to
its parent process.

Author: gfphoenix78

src/bin/pg_autoctl/cli_service.c

@@ -418,6 +418,7 @@ cli_service_stop(int argc, char **argv)
 	{
 		log_fatal("Failed to read the keeper's PID at \"%s\"",
 				  keeper.config.pathnames.pid);
+		exit(EXIT_CODE_INTERNAL_ERROR);
 	}
 }