fix: Error message for incompatible constructor options

[dnlbauer]

The constructor arguments init and passing a jsonld dictionary as source are mutually exclusive.

At the moment, when passing a dict containing JSONLD as source to the contructor and setting init=True, a cryptic error message is shown:

from rocrate.rocrate import ROCrate
crate = ROCrate({"@context": "http://example.com", "@graph": []}, init=True)
Traceback (most recent call last):
  File "<stdin>", line 1, in <module>
  File "/home/dbauer/projects/ro-crate-py/rocrate/rocrate.py", line 94, in __init__
    self.__init_from_tree(source, gen_preview=gen_preview)
  File "/home/dbauer/projects/ro-crate-py/rocrate/rocrate.py", line 101, in __init_from_tree
    top_dir = Path(top_dir)
  File "/home/dbauer/.local/share/uv/python/cpython-3.9.21-linux-x86_64-gnu/lib/python3.9/pathlib.py", line 1082, in __new__
    self = cls._from_parts(args, init=False)
  File "/home/dbauer/.local/share/uv/python/cpython-3.9.21-linux-x86_64-gnu/lib/python3.9/pathlib.py", line 707, in _from_parts
    drv, root, parts = self._parse_args(args)
  File "/home/dbauer/.local/share/uv/python/cpython-3.9.21-linux-x86_64-gnu/lib/python3.9/pathlib.py", line 691, in _parse_args
    a = os.fspath(a)
TypeError: expected str, bytes or os.PathLike object, not dict

This PR replaces this error with a message telling the user what the issue actually is:

Traceback (most recent call last):
  File "<stdin>", line 1, in <module>
  File "/home/dbauer/projects/ro-crate-py/rocrate/rocrate.py", line 95, in __init__
    raise ValueError("parameter init is not compatible with a JsonLD source")
ValueError: parameter init is not compatible with a JsonLD source