OMERO.tables client: provide clearer warning if Tables is disabled

[manics]

From https://forum.image.sc/t/error-creating-table-on-local-server-installation/35697

If OMERO.tables is disabled a user trying to use it sees an unhelpful error:

omero.InternalException: exception ::omero::InternalException
{
    serverStackTrace = 
    serverExceptionClass = 
    message = null table as argument
}

Could we have a more informative error message?