lib.backup() fails only once every worker has stopped

A worker that threw, as one does when onProgress throws on a FAILED line,
made lib.backup() reject at once, so the lock was removed while the other
workers still wrote originals into the backup. The backup now waits for
every worker to settle and then throws the first error found. The README
no longer says thumbnails are fetched one at a time, which no test checks.

Model: opus-5-5

README.md

@@ -1086,8 +1086,7 @@ in-flight fetches by key, and an idle pool never lends its slots to a busy one.
 `lib.backup()`, and so `quak backup`, works on up to the content pool's limit of
 originals at once, 5 unless the library was opened with other `pools`. The limit
 covers everything it does for one original, hashing it for `verify` included, so
-however large the account is, no more originals than that are open at once. With
-`includeThumbnails`, it fetches the thumbnails one at a time.
+however large the account is, no more originals than that are open at once.
 
 ### On-disk cache layout