wip: Propagate failed challenge up and cancel write

* Cancel the write if the database encountered a write error (i.e.
  failed challenge response). This prevents previously experienced
  corruption.
* Need to look at a better way to do this so a proper error is
  propagated back to the user.  Instead "Unknown error" is displayed.
* If a removed Yubikey is to blame, re-inserting the Yubikey won't
  resolve the issue.  Hot plug isn't supported at this point.