Gracefully handle direct URLs for confidential work items

Summary

When loading a confidential item, the error page is not clear.

Screen_Shot_2022-08-16_at_8.55.20_AM

Steps to reproduce

  1. Create a confidential work item
  2. Navigate to it as a user who should not be able to see it (non member or guest project member).

Example Project

https://gitlab.com/gitlab-org/gitlab/-/work_items/113288488

What is the current bug behavior?

See screenshot

What is the expected correct behavior?

Error saying that work item can't be found. Consistent with what we do for issues.

Relevant logs and/or screenshots

Output of checks

Results of GitLab environment info

Expand for output related to GitLab environment info 
(For installations with omnibus-gitlab package run and paste the output of:
`sudo gitlab-rake gitlab:env:info`)

(For installations from source run and paste the output of:
`sudo -u git -H bundle exec rake gitlab:env:info RAILS_ENV=production`)

Results of GitLab application Check

Expand for output related to the GitLab application check 
(For installations with omnibus-gitlab package run and paste the output of:
sudo gitlab-rake gitlab:check SANITIZE=true)


(For installations from source run and paste the output of:
sudo -u git -H bundle exec rake gitlab:check RAILS_ENV=production SANITIZE=true)


(we will only investigate if the tests are passing)

Possible fixes