meta-extract: Catch non-existing paths earlier

[adswa]

When I attempt a metadata extraction of a non-existing file with a relative path, the first thing that returns an error is a get_file_info complaining about a missing "dataset status for dataset":

(handbook) adina@muninn in /tmp/ds1 on git:master!
❱ datalad meta-extract -d . metalad_example_file someo.txt
[ERROR  ] no dataset status for dataset: /tmp/ds1 file: /tmp/ds1/someo.txt 

The logic below seems to be careful to not resolve paths:

datalad-metalad/datalad_metalad/extract.py

Lines 267 to 287 in fc20e5c

	# Create a relative Path-instance, if path to a file is given. We have
	# to be careful not to resolve the path, because that could resolve the
	# git-annex link.
	path_object = None
	if path is not None:
	path_object = Path(path)
	if path_object.is_absolute():
	relative_path = None
	for dataset_path in (source_dataset.pathobj,
	source_dataset.pathobj.resolve()):
	try:
	relative_path = path_object.relative_to(dataset_path)
	break
	except ValueError:
	pass
	if relative_path is None:
	raise ValueError(
	f"The provided path {path} is not contained in the "
	f"dataset given by {source_dataset.pathobj}"
	)
	path_object = relative_path

I think we should check early on if a path points to an existing file, and if not, return a proper error or impossible result early and informatively.