Better group-member handling on non-existent paths

[johnkerl]

Before:

[email protected][prod][python]$ peek-soma nonesuch
>>> soma.obsp.keys()
Traceback (most recent call last):
  File "/Users/johnkerl/git/single-cell-data/TileDB-SingleCell/apis/python/src/tiledbsc/tiledb_group.py", line 84, in _open
    G = self._open_withlessly(mode)
  File "/Users/johnkerl/git/single-cell-data/TileDB-SingleCell/apis/python/src/tiledbsc/tiledb_group.py", line 72, in _open_withlessly
    return tiledb.Group(self.uri, mode=mode, ctx=self._ctx)
  File "/Library/Frameworks/Python.framework/Versions/3.9/lib/python3.9/site-packages/tiledb/group.py", line 188, in __init__
    super().__init__(cctx, uri, query_type)
tiledb.cc.TileDBError: [TileDB::Group] Error: Cannot open group; Group does not exist

During handling of the above exception, another exception occurred:

Traceback (most recent call last):
  File "<stdin>", line 1, in <module>
  File "/Users/johnkerl/git/single-cell-data/TileDB-SingleCell/apis/python/src/tiledbsc/annotation_pairwise_matrix_group.py", line 47, in keys
    return self._get_member_names()
  File "/Users/johnkerl/git/single-cell-data/TileDB-SingleCell/apis/python/src/tiledbsc/tiledb_group.py", line 126, in _get_member_names
    return list(self._get_member_names_to_uris().keys())
  File "/Users/johnkerl/git/single-cell-data/TileDB-SingleCell/apis/python/src/tiledbsc/tiledb_group.py", line 140, in _get_member_names_to_uris
    with self._open("r") as G:
  File "/Library/Frameworks/Python.framework/Versions/3.9/lib/python3.9/contextlib.py", line 117, in __enter__
    return next(self.gen)
  File "/Users/johnkerl/git/single-cell-data/TileDB-SingleCell/apis/python/src/tiledbsc/tiledb_group.py", line 87, in _open
    G.close()
UnboundLocalError: local variable 'G' referenced before assignment

It's the UnboundLocalError: local variable 'G' referenced before assignment part which is super-confusing.

After:

[email protected][prod][python]$ peek-soma nonesuch
>>> soma.obsp.keys()
Traceback (most recent call last):
  File "<stdin>", line 1, in <module>
  File "/Users/johnkerl/git/single-cell-data/TileDB-SingleCell/apis/python/src/tiledbsc/annotation_pairwise_matrix_group.py", line 47, in keys
    return self._get_member_names()
  File "/Users/johnkerl/git/single-cell-data/TileDB-SingleCell/apis/python/src/tiledbsc/tiledb_group.py", line 135, in _get_member_names
    return list(self._get_member_names_to_uris().keys())
  File "/Users/johnkerl/git/single-cell-data/TileDB-SingleCell/apis/python/src/tiledbsc/tiledb_group.py", line 149, in _get_member_names_to_uris
    with self._open("r") as G:
  File "/Library/Frameworks/Python.framework/Versions/3.9/lib/python3.9/contextlib.py", line 117, in __enter__
    return next(self.gen)
  File "/Users/johnkerl/git/single-cell-data/TileDB-SingleCell/apis/python/src/tiledbsc/tiledb_group.py", line 90, in _open
    raise Exception(f"Does not exist: {self.uri}")
Exception: Does not exist: nonesuch/obsp