Add data warn if frozen

[mtmail]

For #3494

$ nominatim import --osm-file country1.pbf --no-updates
$ nominatim add-data --file country2.pbf
Using project directory: /home/vagrant/nominatim-project
Database is marked frozen. New data can't be added.

The documentation seems to cover it already: Warning: The data structure for updates are also required when adding additional data after the import, for example [TIGER housenumber data](https://nominatim.org/release-docs/develop/customize/Tiger/). If you plan to use those, you must not use the --no-updates parameter. Do a normal import, add the external data and once you are done with everything run nominatim freeze.

I added a new pylint exception. Happy to restructure the code instead or limit the exception to just that file if you have ideas.