netcmd: Better error message for backup with no RID pool

netcmd: Better error message for backup with no RID pool

Add a better error message (and what to do about it) if the user tries
to back up a DC that hasn't initialized its RID pool yet.

Seems to be a fairly common problem hit by users.

BUG: https://bugzilla.samba.org/show_bug.cgi?id=14048
RN: Added more informative error message if the 'samba-tool domain
backup' command fails due to no RID pool being present on the DC.

Signed-off-by: Tim Beale <timbeale@catalyst.net.nz>
Reviewed-by: Andrew Bartlett <abartlet@samba.org>
Autobuild-User(master): Andrew Bartlett <abartlet@samba.org>
Autobuild-Date(master): Wed Jul 24 07:07:01 UTC 2019 on sn-devel-184