Modify ExportState in `bbs2gh` so if it treats unexpected values as errors

[dylan-smith]

I managed to get into an infinite loop because I had an invalid value (in this case null), that I was passing to ExportState.IsInProgress() and that treats any unexpected value as InProgress. It should treat unexpected values as errors like we do in other similar places in the code. IsInProgress should specifically check for what state value it expects, and treat anything else as error state.