`llm models` doesn't gracefully handle http connection refused

Steps to reproduce:

1. Create a clean virtual environment
2. pip install llm llm-ollama
3. llm models
4. See the following output (at time of writing, at least):

root@8496f01673ca:/# llm models
Traceback (most recent call last):
  File "/usr/local/lib/python3.12/site-packages/httpx/_transports/default.py", line 66, in map_httpcore_exceptions
    yield
  File "/usr/local/lib/python3.12/site-packages/httpx/_transports/default.py", line 228, in handle_request
    resp = self._pool.handle_request(req)
           ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
  File "/usr/local/lib/python3.12/site-packages/httpcore/_sync/connection_pool.py", line 216, in handle_request
    raise exc from None
  File "/usr/local/lib/python3.12/site-packages/httpcore/_sync/connection_pool.py", line 196, in handle_request
    response = connection.handle_request(
               ^^^^^^^^^^^^^^^^^^^^^^^^^^
  File "/usr/local/lib/python3.12/site-packages/httpcore/_sync/connection.py", line 99, in handle_request
    raise exc
  File "/usr/local/lib/python3.12/site-packages/httpcore/_sync/connection.py", line 76, in handle_request
    stream = self._connect(request)
             ^^^^^^^^^^^^^^^^^^^^^^
  File "/usr/local/lib/python3.12/site-packages/httpcore/_sync/connection.py", line 122, in _connect
    stream = self._network_backend.connect_tcp(**kwargs)
             ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
  File "/usr/local/lib/python3.12/site-packages/httpcore/_backends/sync.py", line 205, in connect_tcp
    with map_exceptions(exc_map):
  File "/usr/local/lib/python3.12/contextlib.py", line 158, in __exit__
    self.gen.throw(value)
  File "/usr/local/lib/python3.12/site-packages/httpcore/_exceptions.py", line 14, in map_exceptions
    raise to_exc(exc) from exc
httpcore.ConnectError: [Errno 111] Connection refused

The above exception was the direct cause of the following exception:

Traceback (most recent call last):
  File "/usr/local/bin/llm", line 8, in <module>
    sys.exit(cli())
             ^^^^^
  File "/usr/local/lib/python3.12/site-packages/click/core.py", line 1157, in __call__
    return self.main(*args, **kwargs)
           ^^^^^^^^^^^^^^^^^^^^^^^^^^
  File "/usr/local/lib/python3.12/site-packages/click/core.py", line 1078, in main
    rv = self.invoke(ctx)
         ^^^^^^^^^^^^^^^^
  File "/usr/local/lib/python3.12/site-packages/click/core.py", line 1688, in invoke
    return _process_result(sub_ctx.command.invoke(sub_ctx))
                           ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
  File "/usr/local/lib/python3.12/site-packages/click/core.py", line 1688, in invoke
    return _process_result(sub_ctx.command.invoke(sub_ctx))
                           ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
  File "/usr/local/lib/python3.12/site-packages/click/core.py", line 1434, in invoke
    return ctx.invoke(self.callback, **ctx.params)
           ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
  File "/usr/local/lib/python3.12/site-packages/click/core.py", line 783, in invoke
    return __callback(*args, **kwargs)
           ^^^^^^^^^^^^^^^^^^^^^^^^^^^
  File "/usr/local/lib/python3.12/site-packages/llm/cli.py", line 799, in models_list
    for model_with_aliases in get_models_with_aliases():
                              ^^^^^^^^^^^^^^^^^^^^^^^^^
  File "/usr/local/lib/python3.12/site-packages/llm/__init__.py", line 80, in get_models_with_aliases
    pm.hook.register_models(register=register)
  File "/usr/local/lib/python3.12/site-packages/pluggy/_hooks.py", line 501, in __call__
    return self._hookexec(self.name, self._hookimpls.copy(), kwargs, firstresult)
           ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
  File "/usr/local/lib/python3.12/site-packages/pluggy/_manager.py", line 119, in _hookexec
    return self._inner_hookexec(hook_name, methods, kwargs, firstresult)
           ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
  File "/usr/local/lib/python3.12/site-packages/pluggy/_callers.py", line 138, in _multicall
    raise exception.with_traceback(exception.__traceback__)
  File "/usr/local/lib/python3.12/site-packages/pluggy/_callers.py", line 102, in _multicall
    res = hook_impl.function(*args)
          ^^^^^^^^^^^^^^^^^^^^^^^^^
  File "/usr/local/lib/python3.12/site-packages/llm_ollama.py", line 26, in register_models
    for model in ollama.list()["models"]:
                 ^^^^^^^^^^^^^
  File "/usr/local/lib/python3.12/site-packages/ollama/_client.py", line 313, in list
    return self._request('GET', '/api/tags').json()
           ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
  File "/usr/local/lib/python3.12/site-packages/ollama/_client.py", line 53, in _request
    response = self._client.request(method, url, **kwargs)
               ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
  File "/usr/local/lib/python3.12/site-packages/httpx/_client.py", line 814, in request
    return self.send(request, auth=auth, follow_redirects=follow_redirects)
           ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
  File "/usr/local/lib/python3.12/site-packages/httpx/_client.py", line 901, in send
    response = self._send_handling_auth(
               ^^^^^^^^^^^^^^^^^^^^^^^^^
  File "/usr/local/lib/python3.12/site-packages/httpx/_client.py", line 929, in _send_handling_auth
    response = self._send_handling_redirects(
               ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
  File "/usr/local/lib/python3.12/site-packages/httpx/_client.py", line 966, in _send_handling_redirects
    response = self._send_single_request(request)
               ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
  File "/usr/local/lib/python3.12/site-packages/httpx/_client.py", line 1002, in _send_single_request
    response = transport.handle_request(request)
               ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
  File "/usr/local/lib/python3.12/site-packages/httpx/_transports/default.py", line 227, in handle_request
    with map_httpcore_exceptions():
  File "/usr/local/lib/python3.12/contextlib.py", line 158, in __exit__
    self.gen.throw(value)
  File "/usr/local/lib/python3.12/site-packages/httpx/_transports/default.py", line 83, in map_httpcore_exceptions
    raise mapped_exc(message) from exc
httpx.ConnectError: [Errno 111] Connection refused

Ideally, this show be a non-critical error, since it's an issue connecting to just one of several possible llm-providers, and it doesn't tell me which one; I had to uninstall modules individually until the problem went away.

Start with the models_list path in llm/cli.py and the register_models function in llm_ollama.py shown in the traceback. Reproduce llm models with Ollama unavailable, then inspect how provider registration errors are handled. Done means the command reports or skips the unavailable provider without aborting model listing or displaying an opaque traceback.