Allow ENOENT in check_mode_recursive().

Allow ENOENT in check_mode_recursive().

Buildfarm member tern failed src/bin/pg_ctl/t/001_start_stop.pl when a
check_mode_recursive() call overlapped a server's startup-time deletion
of pg_stat/global.stat.  Just warn.  Also, include errno in the message.
Back-patch to v11, where check_mode_recursive() first appeared.