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

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.