Cleanup: rename return_code to status_code and fss_return_code to fss_status_code

Cleanup: rename return_code to status_code and fss_return_code to fss_status_code

The "return codes" were originally intended to be literal return codes.
When the error codes were converted to have error bits, warning bits, and signal bits, this no longer became the case.
Refactor return_code into status_code to be more accurate.