Add check to ensure OPA executable is available

[adhilto]

💡 Summary

Before attempting to execute OPA, first check to ensure the executable is available and at least the minimum version, and provide a helpful error message if not.

Motivation and context

If you try running ScubaGoggles without the executable, the error message does not provide the user with actionable information.

Implementation notes

See https://github.com/cisagov/ScubaGoggles/issues/410.

Acceptance criteria

• [ ] Check to see if executable is present is implemented
• [ ] Check to see if acceptable version is implemented
• [ ] Error message contains actionable information