Prevent warden errors in session middleware

What does this MR do and why?

Ignore warden errors in UnauthenticatedSessionExpiry middleware. We don't need to handle these errors since we're just interested in whether a session is authenticated or not so we can set the correct expiry time.

This caused flaky specs as seen in gitlab-org/quality/engineering-productivity/master-broken-incidents#4891 (closed)

Related to #437284 (closed)

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How to set up and validate locally

The flaky failure can be reproduced by applying this diff:

diff --git a/spec/features/issues/issue_detail_spec.rb b/spec/features/issues/issue_detail_spec.rb
index 15cb1678c552..35dd84a3531d 100644
--- a/spec/features/issues/issue_detail_spec.rb
+++ b/spec/features/issues/issue_detail_spec.rb
@@ -109,6 +109,8 @@

       Users::DestroyService.new(user_to_be_deleted).execute(user_to_be_deleted)

+      sleep 3
+
       sign_in(user)
       visit project_issue_path(project, issue)
     end

Then running:

bin/rspec ./spec/features/issues/issue_detail_spec.rb:116

The failure happens because after the user is deleted, the ActionCable request on the current page with deleted user signed in will fail with the 500. Without the sleep, sometimes the spec is fast enough to load the next page that the 500 does not happen.