Since we interrupt all opCtx created on behalf of POS services when the node steps down, OplogBufferCollection methods (used internally by POS services, such as tenant migration and merge) can trigger fasserts, as seen in the BF. Instead of fasserting, throw an exception and let the caller handle those errors.

As part of this ticket, audit other fassert places in oplog_buffer_collection.cpp file to see if they need a similar treatment.