'Check that you can get a port group.
:param serial_number: the array serial number
:param portgroup_name: the port group name
:returns: string -- msg, the error message'

def _check_port_group(self, serial_number, portgroup_name):

    msg = None
    portgroup = self.rest.get_portgroup(serial_number, portgroup_name)
    if (portgroup is None):
        msg = ('Cannot    get    port    group:    %(portgroup)s    from    the    array    %(array)s.    Portgroups    must    be    pre-configured    -    please    check    the    array.' % {'portgroup': portgroup_name, 'array': serial_number})
        LOG.error(msg)
    return (portgroup_name, msg)
