Improve error handling in Problem.from_yaml - format_version

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

--> 250 if yaml_config[FORMAT_VERSION] not in {"1", 1, "1.0.0", "2.0.0"}:
    251     raise ValueError("Provided PEtab files are of unsupported version "
    252                      f"{yaml_config[FORMAT_VERSION]}. Expected "
    253                      f"{format_version.__format_version__}.")
    254 if yaml_config[FORMAT_VERSION] == "2.0.0":

KeyError: 'format_version'

Start at the Problem.from_yaml entry point and reproduce the failure with a YAML file missing format_version. Trace the existing version validation and error handling, then make the missing field produce a proper error instead of KeyError; done means the reported case has a clear error and the relevant checks pass.