Stop execution on invalid OS

🗣 Description

Stop execution if an invalid OS is encountered (to fix linter error).

💭 Motivation and context

Pylint was warning that url and filename on line 189 (now line 190) might not be defined. This is because url and filename are not defined in the preceding else condition. Stopping execution in this case resolves the issue.

🧪 Testing

Ran download_opa.py after hardcoding args.os to an invalid value. Note that simply specifying an invalid choice via the commandline won't work because argparse already forces you to choose a valid choice...meaning this whole fix is really a defensive programming thing.

✅ Pre-approval checklist

[x] This PR has an informative and human-readable title.
[x] Changes are limited to a single goal - eschew scope creep!
[x] If applicable, All future TODOs are captured in issues, which are referenced in the PR description.
[x] The relevant issues PR resolves are linked preferably via closing keywords.
[x] All relevant type-of-change labels have been added.
[x] I have read and agree to the CONTRIBUTING.md document.
[x] These code changes follow cisagov code standards.
[x] All relevant repo and/or project documentation has been updated to reflect the changes in this PR.
[x] Tests have been added and/or modified to cover the changes in this PR.
[x] All new and existing tests pass.

✅ Pre-merge Checklist

[x] This PR has been smoke tested to ensure main is in a functional state when this PR is merged.
[x] Squash all commits into one PR level commit using the Squash and merge button.

✅ Post-merge Checklist

[x] Delete the branch to clean up.
[x] Close issues resolved by this PR if the closing keywords did not activate.

cisagov / ScubaGoggles