netcmd: Try to improve domain backup error message

netcmd: Try to improve domain backup error message

I ran this command as non-root by mistake and didn't find the error
message particularly helpful. Tweak the error message so it reminds the
user that they should be root. Also display the path we're looking for
the sam.ldb file in, to give them more clues.

BUG: https://bugzilla.samba.org/show_bug.cgi?id=13676

Signed-off-by: Tim Beale <timbeale@catalyst.net.nz>
Reviewed-by: Noel Power <npower@samba.org>
Autobuild-User(master): Noel Power <npower@samba.org>
Autobuild-Date(master): Mon Jan 21 16:34:06 CET 2019 on sn-devel-144