An activity call to a non-existent function should error-out

Same as in: https://github.com/Azure/azure-functions-durable-js/issues/197
This appears to be a shared problem among out-of-process languages. Please defer the discussion to the linked thread above

Start with the linked azure-functions-durable-js issue, which this report identifies as the discussion to follow. Confirm the shared out-of-process behavior and determine the expected error for calling a non-existent activity; done means that behavior is defined and consistently handled in the Python library.

Written by the indexing model from the issue text.