[Python] Fine-grained exception class for "recipient not found" in Core

[auto-d]

What I did

Sent a message to an agent before the runtime had actually been created. Caught an Exception object from this line as neither my agent or the runtime had been properly set up.

What I was expecting

A more specific Exception object that I could target to handle this case. I currently need to check for this specific error string to avoid catching every Exception that might by raised by the runtime.

[jackgerrits]

Can you please provide a code example of what you are describing?

[auto-d]

Wow this a terrible write-up. I'm sorry, I tried to create the issue from the offending source line and thought a link would be included here, making the exception in question obvious.

Steps to reproduce

1. Copy the modifier/checker example in the quickstart
2. Comment one of the agent registration calls like this:

# Create an local embedded runtime.
runtime = SingleThreadedAgentRuntime()

# Register the modifier and checker agents by providing
# their agent types, the factory functions for creating instance and subscriptions.
await Modifier.register(
    runtime,
    "modifier",
    # Modify the value by subtracting 1
    lambda: Modifier(modify_val=lambda x: x - 1),
)

# await Checker.register(
#     runtime,
#     "checker",
#     # Run until the value is less than or equal to 1
#     lambda: Checker(run_until=lambda x: x <= 1),
# )

# Start the runtime and send a direct message to the checker.
runtime.start()
await runtime.send_message(Message(10), AgentId("checker", "default"))
await runtime.stop_when_idle()            

3. Receive a vanilla Exception with description "Recipient not found"

[jackgerrits]

Thanks for providing more info! Yeah the one you referenced is not good at all. This is good to fix and best thing is (IMO) this is not breaking as our other custom exceptions also inherit from Exception