xrootdpyfs: fix extended attributes fetch

[alejandromumo]

Problem

After an upload fails we try to delete the file(s) from the FS. This operation eventually checks whether the file is a directory (using the fs getinfo method). In turn, this executes fs._query(QueryCode.XATTR, path) which is not a valid query for XATTR (it seems it's an unsupported operation).

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Possible fix

From what I see, the client supports extended attributes listing which I believe would be what we need. I ran some tests in the CLI and seems to be OK:

from xrootdpyfs.fs import XRootDPyFS

url = '...'
fs = XRootDPyFS(url)
path = '...'

# Current code that fails
fs._query(QueryCode.XATTR, path)

# Alternative code
extended_attr = fs._client.list_xattr(path)

In the example above, I realised that we have another issue when raising the fs exceptions (e.g. Unsupported). We must use raise Unsupported(msg=status.message). This is also being tracked by #122

If this is fixed, we could potentially have more visibility on why the upload failed on the first place (to be tested)