General error recovery/handling behavior

[natevw]

When anything goes wrong, we basically just bubble it up the stack. But what state does this leave the on-disk structures? Or our in-memory structures vs. the on-disk ones?

When it made sense, I've usually taken care to do on-disk operations in the "safest" (i.e. most interruptible) order. But maybe not always, and there's also state like _fd.entry._size that could get mismatched.