Improving error messages

[chStaiger]

Related to #338

The error messages in the step create-meta do not specify which variable is causing the trouble.
It would be nice to print the variable name and to format the error message a bit.

Example error when employing a config file with more than 10 variables :(

staig001@busoni create_syn_data %  metasyn create-meta --config environment_config.toml -o environment_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 147, in fit_dataframe
    distribution = meta_config.dist_providers.create(var_spec)
                   ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
  File "/Users/staig001/git-repos/metasyn/metasyn/provider.py", line 249, in create
    return dist_class(**dist_spec.parameters)
           ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
TypeError: UniqueKeyDistribution.__init__() got an unexpected keyword argument 'upper'