Introduce `ParameterMissingError` and `ParameterInvalidError` structs, to
be returned from command executors. These replace free-form `fmt.Errorf()`
style errors.
Fix the FFmpeg unit test on Windows, by:
- Having actual input files (otherwise the input-glob-creation-function
errors out), and
- ensuring the cleanup function is always run, and
- testing for the right CLI arguments.
