Make dashboard and project page resilient to missing filesystem or Gitaly access

This is a follow up from https://gitlab.com/gitlab-com/infrastructure/issues/3177

On staging we don't have the complete filesystem as production, and due to that a lot of pages are just broken on staging.

This is the error for a project page without having a working filesystem: https://sentry.gitlap.com/gitlab/staginggitlabcom/issues/102971/

This is the error for my dashboard: https://sentry.gitlap.com/gitlab/staginggitlabcom/issues/104597/

If we could just show some nice error messages, we could make staging work again.