registry: missing err key in async_wrapper entry

[rokm]

#1794 added async_wrapper to the known_implementations in registry, but did so without an error message (the err element):

https://github.com/fsspec/filesystem_spec/blob/master/fsspec/registry.py#L75-L77

Therefore, trying to list available/working protocols using the following code snippet:

import fsspec

working_protocols = []
for protocol in fsspec.available_protocols():
    try:
        fsspec.get_filesystem_class(protocol)
        working_protocols.append(protocol)
    except ImportError:
        pass

print(working_protocols)

fails with 2025.3.0 when morefs is not installed, whereas it used to work in earlier versions. That's because the lack of err ends up triggering a KeyError instead of an ImportError:

Traceback (most recent call last):
  File "[...]/lib64/python3.13/site-packages/fsspec/registry.py", line 250, in get_filesystem_class
    register_implementation(protocol, _import_class(bit["class"]))
                                      ~~~~~~~~~~~~~^^^^^^^^^^^^^^
  File "[...]/lib64/python3.13/site-packages/fsspec/registry.py", line 286, in _import_class
    mod = importlib.import_module(mod)
  File "/usr/lib64/python3.13/importlib/__init__.py", line 88, in import_module
    return _bootstrap._gcd_import(name[level:], package, level)
           ~~~~~~~~~~~~~~~~~~~~~~^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
  File "<frozen importlib._bootstrap>", line 1387, in _gcd_import
  File "<frozen importlib._bootstrap>", line 1360, in _find_and_load
  File "<frozen importlib._bootstrap>", line 1310, in _find_and_load_unlocked
  File "<frozen importlib._bootstrap>", line 488, in _call_with_frames_removed
  File "<frozen importlib._bootstrap>", line 1387, in _gcd_import
  File "<frozen importlib._bootstrap>", line 1360, in _find_and_load
  File "<frozen importlib._bootstrap>", line 1324, in _find_and_load_unlocked
ModuleNotFoundError: No module named 'morefs'

During handling of the above exception, another exception occurred:

Traceback (most recent call last):
  File "list_working_protocols.py", line 6, in <module>
    fsspec.get_filesystem_class(protocol)
    ~~~~~~~~~~~~~~~~~~~~~~~~~~~^^^^^^^^^^
  File "[...]/lib64/python3.13/site-packages/fsspec/registry.py", line 253, in get_filesystem_class
    raise ImportError(bit["err"]) from e
                      ~~~^^^^^^^
KeyError: 'err'

[martindurant]

Fixed in #1806