Logging or Exceptions when Durable Functions Activity Does Not Exist

Pretty simple request, but not sure how simple it would be to implement.

Basically I would like some sort of feedback when you use the call_activity() method of the DurableOrchestrationContext class with a Durable Function Activity that does not exist.

```python
import azure.functions as func
import azure.durable_functions as df

def orchestrator_function(context: df.DurableOrchestrationContext):
logging.info("Starting Activity!")
result = yield context.call_activity("I_Do_Not_Exist", None, None)
logging.info("You will never see me because the code stops at the above step!")
return result

main = df.Orchestrator.create(orchestrator_function)
```

Side note: I may or may not have spent an hour and a half troubleshooting an orchestrator only to realize it was a typo in the activity name.

Start at DurableOrchestrationContext.call_activity() and trace how an unknown activity name is handled by the library and runtime. Check the existing activity dispatch and error-handling tests; done means a missing activity produces clear feedback instead of stopping without a useful explanation, with regression coverage.

Written by the indexing model from the issue text.