deleteObject() replied 404 for any error from newObject(), while
serveObject() in the same file replies 404 only for
fs.ErrorObjectNotFound and 500 otherwise.
restic treats a 404 on DELETE as final and never retries, so a
transient lookup failure while removing a lock left the lock behind
and broke the next exclusive operation (eg forget --prune failing with
"repository is already locked").
This change mirrors serveObject() handling of 404/500 errors in
deleteObject().
Fixes#10100
Co-authored-by: bunnysayzz <bunnysayzz@users.noreply.github.com>
