Improving error message for prop_missing

[chStaiger]

The current error message when one forgets to set the prop_missing parameter in the config file is

staig001@busoni create_syn_data % metasyn create-meta --config host_config.toml -o hosts_gmf_1
.json
Traceback (most recent call last):
  File "/opt/homebrew/bin/metasyn", line 8, in <module>
    sys.exit(main())
             ^^^^^^
  File "/Users/staig001/git-repos/metasyn/metasyn/__main__.py", line 82, in main
    create_metadata()
  File "/Users/staig001/git-repos/metasyn/metasyn/__main__.py", line 132, in create_metadata
    meta_frame = MetaFrame.from_config(meta_config)
                 ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
  File "/Users/staig001/git-repos/metasyn/metasyn/metaframe.py", line 177, in from_config
    return cls.fit_dataframe(None, meta_config)
           ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
  File "/Users/staig001/git-repos/metasyn/metasyn/metaframe.py", line 148, in fit_dataframe
    var = MetaVar(
          ^^^^^^^^
  File "/Users/staig001/git-repos/metasyn/metasyn/var.py", line 77, in __init__
    if self.prop_missing < -1e-8 or self.prop_missing > 1 + 1e-8:
       ^^^^^^^^^^^^^^^^^^^^^^^^^
TypeError: '<' not supported between instances of 'NoneType' and 'float'

It would be nice if the error message could state for which variable the prop_missing is missing.