Fix exception handling when a concurrent backup fails

If a concurrent backup encounters an exception, we should allow the
current threads to run to completion instead of aborting the whole
backup. This was causing threads to leak outside of the tests in
https://gitlab.com/gitlab-org/gitlab/-/issues/241251 and causing the
query cache to be enabled.

This also reverts the quarantined spec in
f47a2f17.

Closes https://gitlab.com/gitlab-org/gitlab/-/merge_requests/40451

changelogs/unreleased/sh-fix-issue-241251.yml

+---
+title: Fix exception handling when a concurrent backup fails
+merge_request: 40451
+author:
+type: fixed

lib/backup/repository.rb

@@ -182,12 +182,12 @@ module Backup
         queue.push(project)
       end
 
+      raise errors.pop unless errors.empty?
+    ensure
       queue.close
       ActiveSupport::Dependencies.interlock.permit_concurrent_loads do
         threads.each(&:join)
       end
-
-      raise errors.pop unless errors.empty?
     end
 
     def dump_project(project)

spec/lib/backup/repository_spec.rb

@@ -19,7 +19,7 @@ RSpec.describe Backup::Repository do
     end
   end
 
-  describe '#dump', quarantine: 'https://gitlab.com/gitlab-org/gitlab/-/issues/241251' do
+  describe '#dump' do
     before do
       allow(Gitlab.config.repositories.storages).to receive(:keys).and_return(storage_keys)
     end