A backup whose lock is lost stops with one line, not a stack trace (closes #184)

proper-lockfile's default onCompromised throws from a timer, so a backup
whose backup.lock was removed or taken over by another backup died with
a stack trace. lockBackupDirectory now returns the lock, its release
function and why it was lost, and passes a handler that only records
the reason. The backup checks it after the refresh and before each
original, thumbnail, file's JSON and album, and before it writes
failures.json, and then fails with "lost the lock on DIR: REASON",
which quak backup prints as one line, exiting 1. lib.backup() takes a
caller's lock as lock, in place of lockHeld: true. Releasing a lost lock
leaves backup.lock alone, since it may be another backup's.

Model: opus-5-5

src/index.ts

@@ -69,6 +69,7 @@ export {
     type EnsureEvent,
     lockBackupDirectory,
     runBackup,
+    type BackupLock,
     type BackupOptions,
     type BackupResult,
     type BackupError,