Improve error handling in Problem.from_yaml - parameter_file

Missing parameter_file in the yaml file should provide a proper error message instead of just a KeyError:

File petab/problem.py:259, in Problem.from_yaml(yaml_config)
    255     warn("Support for PEtab2.0 is experimental!")
    257 problem0 = yaml_config['problems'][0]
--> 259 if isinstance(yaml_config[PARAMETER_FILE], list):
    260     parameter_df = parameters.get_parameter_df([
    261         get_path(f)
    262         for f in yaml_config[PARAMETER_FILE]
    263     ])
    264 else:

KeyError: 'parameter_file'

Also applies to sbml_files in --> 270 if len(problem0[SBML_FILES]) > 1:

Read Problem.from_yaml in petab/problem.py at the parameter_file and sbml_files accesses. Check the surrounding YAML parsing behavior, then exercise inputs missing each key and verify they produce clear errors rather than raw KeyError exceptions.