Marker: crash on question out of range, and wrong range

Wrong question label range displayed. Was off-by-one. Fixes #1859 (closed).

Fixes #1911 (closed). Not so nice a fix: its hard to fail nicely during Marker startup (which is probably not good). I cleaned up the close code to work if there is not yet a backgroupUploader, which helps. #1911 (closed) is regression b/c we used to get a somewhat crytic error message 416 question out of range. Now it does that again; better than a crash.