interrupting --prune-blockchain cancels historical pruning

[nahuhh]

1. Sync blockchain w/o pruning
2. Restart monerod with --prune-blockchain and --log-level 2. Observe the pruning process
3. stop monerod (ctrl c) while still pruning historical blocks
4. Repeat step 2

result = the pruning was interrupted and not continued. It appears as though new blocks are still pruned, but the pruning of old blocks is permanently cancelled

expected either:
a) not able gracefully stop monerod while pruning
b) historical pruning to continue where it left off