netcmd: Improve error handling of gpo aclcheck as non-admin

netcmd: Improve error handling of gpo aclcheck as non-admin

Reading the nTSecurityDescriptor attribute over LDAP requires admin
creds. However, if you don't specify admin creds, then you get an error
like this:

bin/samba-tool gpo aclcheck
ERROR(<class 'KeyError'>): uncaught exception - 'No such element'
  File "bin/python/samba/netcmd/__init__.py", line 184, in _run
    return self.run(*args, **kwargs)
  File "bin/python/samba/netcmd/gpo.py", line 1536, in run
    ds_sd_ndr = m['nTSecurityDescriptor'][0]

This patch adds an explicit check/error message to make the problem
clearer.

Signed-off-by: Tim Beale <timbeale@catalyst.net.nz>
Reviewed-by: Andrew Bartlett <abartlet@samba.org>