Gracefully handle missing vulnerability feedback for non-authenticated users

Summary

When a user is not signed into GitLab security reports continue to load but any missing feedback results in a 404. This results in an error and prevents the MR widget from loading cleanly.

Steps to reproduce

  1. Open gitlab-org/security-products/analyzers/brakeman!29 (merged) in incognito window
  2. Note the MR widget fails to load and check relevant requests

Example Project

gitlab-org/security-products/analyzers/brakeman!29 (merged)

What is the current bug behavior?

errors when loading results shown

What is the expected correct behavior?

Vulnerabilities should be shown

Relevant logs and/or screenshots

Screen_Shot_2020-06-29_at_12.09.38_PM

Output of checks

This bug happens on GitLab.com

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

(If you can, link to the line of code that might be responsible for the problem)