Don't delete user session cookie if redis connection errors

Created by: pjlast

If Redis is unavailable due to something like a server restart, this prevents the user session cookie from being deleted. It is still a valid session, and will be valid as soon as Redis is available again.

This addresses https://github.com/sourcegraph/sourcegraph/issues/37016 , where off-screen tabs that perform a request in the background can cause a user session to be ended during a server update, which happens frequently on the k8s dogfood deployment.

Test plan

Manual testing as its a strange redis availability issue.