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

lockBackupDirectory now passes proper-lockfile an onCompromised that records
why the lock was lost, where the default threw from a timer and ended the
process. The backup checks its lock before each original and before it
writes failures.json, and fails with "lost the lock on DIR: REASON", which
quak backup prints through run as one line before exiting 1. Releasing a
lost lock leaves backup.lock alone, as it may be another backup's. The lock
of each directory is kept by path so that a backup run with lockHeld, as
quak backup runs it, finds the lock its caller took.

Model: opus-5-5

README.md

@@ -761,9 +761,9 @@ drops it.
 `backup.lock` keeps two backups of the same directory from running at once, such
 as a cron run that starts while the previous one is still going. A backup
 creates `<dir>` if it is missing and takes the lock before its refresh, and
-removes the lock when it ends, whether it succeeds or fails; `quak backup` takes
-it before it opens its library, so a refused run sends no request. The lock is a
-directory that
+removes the lock when it ends, whether it succeeds or fails, unless it lost the
+lock (see below); `quak backup` takes it before it opens its library, so a
+refused run sends no request. The lock is a directory that
 [proper-lockfile](https://github.com/moxystudio/node-proper-lockfile) creates
 and keeps touching while the backup runs. A second backup of the directory, from
 another process or the same one, fails at once: `quak backup` prints
@@ -775,6 +775,16 @@ over, so nobody has to remove it. The lock is outside the date folders and
 `collections/`, so it is never taken for an original, and the removal of old
 album directories never touches it.
 
+A running backup loses its lock when someone removes `backup.lock`, or when
+another backup takes it over because the running one left it untouched for 10
+seconds, as a stopped or stalled process does. The backup finds this out the
+next time it goes to touch the lock, which it does every 5 seconds, and then
+stops before it starts another file. The original it is downloading at that
+moment is still stored. It leaves `failures.json` and `backup.lock` as they are,
+since another backup may be using them. `quak backup` prints
+`quak: lost the lock on <dir>: <reason>` as one line and exits 1, and
+`lib.backup()` rejects with `lost the lock on <dir>: <reason>`.
+
 A collection's directory and JSON are named after the collection, and a symlink
 after the file's title, both with unsafe characters replaced. When two
 collections would get the same name, or two symlinks in one collection the same
@@ -1074,7 +1084,9 @@ photos newest first). `lib.subscribe({ onChange })` delivers a `LibraryChange`
   back to the library's), `includeOriginals` (default `true`),
   `includeThumbnails` (default `false`), `onlyAlbumNames`, `verify` (default
   `false`), `onProgress`, and `lockHeld` (default `false`). See Backup layout
-  above for the tree it writes.
+  above for the tree it writes. Whether the backup took the lock or its caller
+  did, once the lock is lost the backup rejects with
+  `lost the lock on <dir>: <reason>` before it starts another file.
 
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