xrootdpyfs: fix extended attributes fetch

## 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](https://github.com/inveniosoftware/xrootdpyfs/blob/403566c8e3dce8ce7f47cdc8c13d863d299e9c11/xrootdpyfs/fs.py#L490)). In turn, this executes `fs._query(QueryCode.XATTR, path)` which is not a valid query for XATTR (it seems it's an unsupported operation).

---

## 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:

```python
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](https://github.com/inveniosoftware/xrootdpyfs/blob/403566c8e3dce8ce7f47cdc8c13d863d299e9c11/xrootdpyfs/fs.py#L173) the fs exceptions (e.g. `Unsupported`). We must use raise `Unsupported(msg=status.message)`. This is also being tracked by https://github.com/inveniosoftware/xrootdpyfs/issues/122

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