Handling policy for unhandled agent exceptions

### What happened?

If a custom agent raises an exception, there is an error in the processing of the message and the agent carries on.

```
ERROR:autogen_core:Error processing publish message
Traceback (most recent call last):
  File "/workspaces/dstoolkit-text2sql-and-imageprocessing/.venv/lib/python3.12/site-packages/autogen_core/_single_threaded_agent_runtime.py", line 410, in _process_publish
    await asyncio.gather(*responses)
  File "/workspaces/dstoolkit-text2sql-and-imageprocessing/.venv/lib/python3.12/site-packages/autogen_core/_single_threaded_agent_runtime.py", line 402, in _on_message
    return await agent.on_message(
           ^^^^^^^^^^^^^^^^^^^^^^^
  File "/workspaces/dstoolkit-text2sql-and-imageprocessing/.venv/lib/python3.12/site-packages/autogen_core/_base_agent.py", line 113, in on_message
    return await self.on_message_impl(message, ctx)
           ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
  File "/workspaces/dstoolkit-text2sql-and-imageprocessing/.venv/lib/python3.12/site-packages/autogen_agentchat/teams/_group_chat/_sequential_routed_agent.py", line 48, in on_message_impl
    return await super().on_message_impl(message, ctx)
           ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
  File "/workspaces/dstoolkit-text2sql-and-imageprocessing/.venv/lib/python3.12/site-packages/autogen_core/_routed_agent.py", line 485, in on_message_impl
    return await h(self, message, ctx)
           ^^^^^^^^^^^^^^^^^^^^^^^^^^^
  File "/workspaces/dstoolkit-text2sql-and-imageprocessing/.venv/lib/python3.12/site-packages/autogen_core/_routed_agent.py", line 268, in wrapper
    return_value = await func(self, message, ctx)  # type: ignore
                   ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
  File "/workspaces/dstoolkit-text2sql-and-imageprocessing/.venv/lib/python3.12/site-packages/autogen_agentchat/teams/_group_chat/_chat_agent_container.py", line 53, in handle_request
    async for msg in self._agent.on_messages_stream(self._message_buffer, ctx.cancellation_token):
  File "/workspaces/dstoolkit-text2sql-and-imageprocessing/text_2_sql/autogen/src/autogen_text_2_sql/custom_agents/sql_schema_selection_agent.py", line 71, in on_messages_stream
    entity_results = await asyncio.gather(*entity_tasks)
                     ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
  File "/workspaces/dstoolkit-text2sql-and-imageprocessing/text_2_sql/text_2_sql_core/src/text_2_sql_core/connectors/open_ai.py", line 49, in run_completion_request
    response = await open_ai_client.beta.chat.completions.parse(
               ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
  File "/workspaces/dstoolkit-text2sql-and-imageprocessing/.venv/lib/python3.12/site-packages/openai/resources/beta/chat/completions.py", line 427, in parse
    return await self._post(
           ^^^^^^^^^^^^^^^^^
  File "/workspaces/dstoolkit-text2sql-and-imageprocessing/.venv/lib/python3.12/site-packages/openai/_base_client.py", line 1843, in post
    return await self.request(cast_to, opts, stream=stream, stream_cls=stream_cls)
           ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
  File "/workspaces/dstoolkit-text2sql-and-imageprocessing/.venv/lib/python3.12/site-packages/openai/_base_client.py", line 1537, in request
    return await self._request(
           ^^^^^^^^^^^^^^^^^^^^
  File "/workspaces/dstoolkit-text2sql-and-imageprocessing/.venv/lib/python3.12/site-packages/openai/_base_client.py", line 1638, in _request
    raise self._make_status_error_from_response(err.response) from None
```

### What did you expect to happen?

Option for all exceptions to be raised and for the group chat to stop further message rounds.

This exception stems from a bad API request, so if this agent fails, the whole system should fail.

### How can we reproduce it (as minimally and precisely as possible)?

Create a custom agent that throws an exception. 
Watch how the group chat will fail to register the message and then carry on. 

### AutoGen version

0.4.dev.13

### Which package was this bug in

AgentChat

### Model used

_No response_

### Python version

_No response_

### Operating system

_No response_

### Any additional info you think would be helpful for fixing this bug

_No response_

Labels: proj-core
State: closed