Remove use of unwrap

[momirza]

Propagates error back instead of panicking. We had an issue in production when a worker in the process of shutting down returned SystemExit when calling inner.instance(py).str() on the Python object. This should be propagated back instead of resulting in a PanicException.

thread '<unnamed>' panicked at 'called `Result::unwrap()` on an `Err` value: PyErr { type: <class 'SystemExit'>, value: SystemExit(1), traceback: Some(<traceback object at 0x7fe1bbcacd70>) }'

[davidhewitt]

Thanks for fixing, sorry for letting this panic slip through review 😬

I'm going to adjust the CHANGELOG message - I prefer them to be more informative to how the typical user experience will change rather than the actual code change we made.

I'll push a commit to tweak and then merge.