fix: Raise exception when tool call result is None

[terrytangyuan]

What does this PR do?

When there are issues with the tool call function, an exception is raised but the error message is not informative. This adds a clearer message to tell users to check their functions.

Traceback (most recent call last):
  File "/Users/phayes/projects/llama-stack/llama-stack/llama_stack/distribution/server/server.py", line 208, in sse_generator
    async for item in event_gen:
  File "/Users/phayes/projects/llama-stack/llama-stack/llama_stack/providers/inline/agents/meta_reference/agents.py", line 165, in _create_agent_turn_streaming
    async for event in agent.create_and_execute_turn(request):
  File "/Users/phayes/projects/llama-stack/llama-stack/llama_stack/providers/inline/agents/meta_reference/agent_instance.py", line 197, in create_and_execute_turn
    async for chunk in self.run(
  File "/Users/phayes/projects/llama-stack/llama-stack/llama_stack/providers/inline/agents/meta_reference/agent_instance.py", line 389, in run
    async for res in self._run(
  File "/Users/phayes/projects/llama-stack/llama-stack/llama_stack/providers/inline/agents/meta_reference/agent_instance.py", line 811, in _run
    content=tool_result.content,
AttributeError: 'NoneType' object has no attribute 'content'

Test Plan

Ran the same script and exception is raised with clearer error message.