DiscIO: Clear error status when reading file

If the error status isn't cleared, getting an error on one read results in getting errors on every read after that, regardless of whether they succeed. This wasn't a problem before 4.0-4585 ([PR 1682](https://github.com/dolphin-emu/dolphin/pull/1682)), because VolumeHandler discarded all errors back then. See [issue 7930](https://code.google.com/p/dolphin-emu/issues/detail?id=7930) for details.