fix: Ensure a better error stack trace when llama-stack is not built (#950)

# 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/python@3.11/3.11.10/Frameworks/Python.framework/Versions/3.11/lib/python3.11/asyncio/runners.py", line 190, in run
    return runner.run(main)
           ^^^^^^^^^^^^^^^^
  File "/opt/homebrew/Cellar/python@3.11/3.11.10/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/python@3.11/3.11.10/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](https://github.com/meta-llama/llama-stack/blob/main/CONTRIBUTING.md),
      Pull Request section?
- [ ] Updated relevant documentation.
- [ ] Wrote necessary unit or integration tests.

---------

Signed-off-by: Charlie Doern <cdoern@redhat.com>
Co-authored-by: Ashwin Bharambe <ashwin.bharambe@gmail.com>

llama_stack/distribution/library_client.py

@@ -198,6 +198,7 @@ class AsyncLlamaStackAsLibraryClient(AsyncLlamaStackClient):
 
     async def initialize(self) -> bool:
         try:
+            self.endpoint_impls = None
             self.impls = await construct_stack(self.config, self.custom_provider_registry)
         except ModuleNotFoundError as _e:
             cprint(_e.msg, "red")