Better error message when missing ccs_config?

[mnlevy1981]

If you checkout CESM and then use manage_externals to checkout cime and a few components but forget to also checkout ccs_config, then create_newcase returns a weird XML error AttributeError: 'NoneType' object has no attribute 'xml_element' (@dabail10 was running into this issue after checking out CIME, MOM6, and CICE but not https://github.com/ESMCI/ccs_config_cesm). I think it would be great if CIME could check for ccs_config at the beginning of create_newcase and raise a more informative FileNotFoundError rather than waiting for the first subroutine looking for ccs_config to fail.