fix: Ensure a better error stack trace when llama-stack is not built

[cdoern]

What does this PR do?

currently this is the output when you run a distribution locally without running llama stack build:

Traceback (most recent call last):
  File "/Users/charliedoern/Documents/llama-sdk.py", line 25, in <module>
    models = client.models.list()
             ^^^^^^^^^^^^^^^^^^^^
  File "/Users/charliedoern/Documents/llama-stack-client-python/src/llama_stack_client/resources/models.py", line 107, in list
    raise exc
  File "/Users/charliedoern/Documents/llama-stack-client-python/src/llama_stack_client/resources/models.py", line 95, in list
    return self._get(
           ^^^^^^^^^^
  File "/Users/charliedoern/Documents/llama-stack-client-python/src/llama_stack_client/_base_client.py", line 1212, in get
    return cast(ResponseT, self.request(cast_to, opts, stream=stream, stream_cls=stream_cls))
                           ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
  File "/Users/charliedoern/Documents/llama-stack/llama_stack/distribution/library_client.py", line 168, in request
    return asyncio.run(self.async_client.request(*args, **kwargs))
           ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
  File "/opt/homebrew/Cellar/[email protected]/3.11.10/Frameworks/Python.framework/Versions/3.11/lib/python3.11/asyncio/runners.py", line 190, in run
    return runner.run(main)
           ^^^^^^^^^^^^^^^^
  File "/opt/homebrew/Cellar/[email protected]/3.11.10/Frameworks/Python.framework/Versions/3.11/lib/python3.11/asyncio/runners.py", line 118, in run
    return self._loop.run_until_complete(task)
           ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
  File "/opt/homebrew/Cellar/[email protected]/3.11.10/Frameworks/Python.framework/Versions/3.11/lib/python3.11/asyncio/base_events.py", line 654, in run_until_complete
    return future.result()
           ^^^^^^^^^^^^^^^
  File "/Users/charliedoern/Documents/llama-stack/llama_stack/distribution/library_client.py", line 258, in request
    if not self.endpoint_impls:
           ^^^^^^^^^^^^^^^^^^^
AttributeError: 'AsyncLlamaStackAsLibraryClient' object has no attribute 'endpoint_impls'

the intended exception is never raised, add an except for an AttributeError so users can catch when they call things like models.list() and so that a more useful error telling them that the client is not properly initialized is printed.

Test Plan

Please describe:

• I ran the script found here: https://llama-stack.readthedocs.io/en/latest/getting_started/index.html#run-inference-with-python-sdk locally with the changes in this PR and the exception was caught successfully.

Before submitting

• This PR fixes a typo or improves the docs (you can dismiss the other checks if that's the case).
• Ran pre-commit to handle lint / formatting issues.
• Read the contributor guideline,
  Pull Request section?
• Updated relevant documentation.
• Wrote necessary unit or integration tests.

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[terrytangyuan]

Thanks! Could you sign the individual CLA?

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[nathan-weinberg]

LGTM

[ashwinb]

I think a better fix is to just initialize endpoint_impls = None in the constructor so that the correct ValueError is raised here instead of a weird AttributeError. Trapping an AttributeError is kind of unpredictable anyhow.

[cdoern]

@terrytangyuan @ashwinb @raghotham is this good to go?