Invalid yaml in a charm's actions.yaml results in cryptic "unexpected error"

This is a charm whose actions.yaml doesn't parse correctly (it's invalid yaml).

The user sees:

2021-10-19 09:05:01.028 Status checked: {'revisions': [{'errors': [{'code': 'review-error', 'message': 'Unexpected error'}], 'revision': None, 'status': 'rejected', 'upload-id': '2d5d38aa-13e4-4bfb-bfee-2455ba5bdb99'}]}

On our side, the exception from packagereview is:

Failed to scan /srv/packagereview-worker/worker-tmp/tmpzsz79z47/2d5d38aa-13e4-4bfb-bfee-2455ba5bdb99
Expected '<document start>', but found '<block sequence start>'
  in "actions.yaml", line 2, column 1

The above is a yaml exception, the thing here is that the charm_scan method has a large block where it tries a few things and catches a few exceptions, but the loading of yaml files (done in line 143):

            extra = extract_optional_files(charm, optional)

is not wrapped in any of the specific exceptions, so any exception in that block falls through to:

    except Exception as exc:
        logger.exception("Failed to scan %s", download_path)
        output = dict(error=str(exc), reason="Unexpected error")
        return ScanStatus.rejected.value, output

It would be good to wrap extract_optional_files in a try: block and raise a more explicit, clearer error that a specific file has yaml parsing issues.