Make Pages disk access fail in web server

Previously, when GITLAB_PAGES_DENY_DISK_ACCESS was set, accessing Gitlab::Pages::Settings.path from a web process would track an exception and continue on.

Now it will simply raise the exception and fail. This is to make it easier for developers to find a problem with their code before taking it to production on GitLab.com, where the Pages NFS mount will soon be removed.

This error hasn't been reported in Sentry since we did #241721 (closed); see https://sentry.gitlab.net/gitlab/gitlabcom/issues/?query=is%3Aunresolved%20DiskAccessDenied

Closes #239350 (closed).