Use & Test Specific Exit Codes in Smoke Tests

[tdurk93]

1. We should start using unique exit codes to differentiate between reasons for exiting (e.g. failing a scan versus versus failing due to an exception).
2. Scan failures are the most important case to recognize and are already implemented (they now return a 3) but there may be others cases we wish to distinguish/test
3. We aren't currently testing for specific exit codes in any of our pipeline's post-release smoke tests, only checking for a nonzero response. We should be checking for specific exit codes.
4. We should be utilizing the ExitCode enum added in the models/ directory.
5. We should also be testing for a "happy-path" scan that returns a 0 response code (note: will either require staging some files and scanning the newly-staged diffs, or adding the ability to scan a specific subdirectory prior to completing this work)