FileStructure.BOMGarbageNoWebVTT -- Mistakenly expected to parse?

```
If line is less than six characters long, then abort these steps. The file does not start with the correct WebVTT file signature and was therefore not successfully processed.
```

I don't think this should be expected to parse. Even though this rule is from the "parsing section", it is at least some protection against very fuzzy input.

I should note that like everything else, the error callback is supposed to be able to have some control over which errors are treated as fatal and which ones aren't (currently, that control is very minimal) -- for now we abort parsing as soon as we return a WEBVTT_MALFORMED_TAG error.