clarify error messages from config input errors

[pingings]

The default config file contains invalid default values for experiment.id and participant.id which throws an error, however the problem could be made more clear

Example of error message:

Traceback (most recent call last):
  File "C:\Users\Admin\.conda\envs\matbii\lib\runpy.py", line 196, in _run_module_as_main
    return _run_code(code, main_globals, None,
  File "C:\Users\Admin\.conda\envs\matbii\lib\runpy.py", line 86, in _run_code
    exec(code, run_globals)
  File "C:\Users\Admin\.conda\envs\matbii\lib\site-packages\matbii\__main__.py", line 78, in <module>
    config = Configuration.from_file(args.config, context=context)
  File "C:\Users\Admin\.conda\envs\matbii\lib\site-packages\matbii\config\__init__.py", line 263, in from_file
    return Configuration.model_validate_json(f.read(), context=context)
  File "C:\Users\Admin\.conda\envs\matbii\lib\site-packages\pydantic\main.py", line 597, in model_validate_json
    return cls.__pydantic_validator__.validate_json(json_data, strict=strict, context=context)
pydantic_core._pydantic_core.ValidationError: 2 validation errors for Configuration
experiment.id
  Input should be a valid string [type=string_type, input_value=None, input_type=NoneType]
    For further information visit https://errors.pydantic.dev/2.8/v/string_type
participant.id
  Input should be a valid string [type=string_type, input_value=None, input_type=NoneType]
    For further information visit https://errors.pydantic.dev/2.8/v/string_type