This fixes future issues like bug #3614654:
"ALL: ScummVM 1.5.0 can't read newer saved games".
There are a few behavior changes introduced with this commit:
- checkThumbnailHeader will now also report the presence of
unsupported/broken (but skippable) headers.
- skipThumbnail will also try to skip the data for broken/unsupported
thumbnail data.
- loadThumbnail will skip over broken/unsupported thumbnail data but still
return 0 in this case.