FFS-1427: Implement: Error page for 404/500 errors

[tdooner]

Ticket

Resolves FFS-1427.

Changes

FFS-1427: Implement custom 404/500 errors

Context for reviewers

Acceptance testing

• No acceptance testing needed
  • This change will not affect the user experience (bugfix, dependency updates, etc.)
• Acceptance testing prior to merge
  • This change can be verified visually via screenshots attached below or by sending a link to a local development environment to the acceptance tester
  • Acceptance testing should be done by design for visual changes, product for behavior/logic changes, or both for changes that impact both.
• Acceptance testing after merge
  • This change is hard to test locally, so we'll test it in the demo environment (deployed automatically after merge.)
  • Make sure to notify the team once this PR is merged so we don't inadvertently deploy the unaccepted change to production. (e.g. :alert: Deploy block! @ffs-eng I just merged PR [#123] and will be doing acceptance testing in demo - please don't deploy until I'm finished!)

[tdooner]

this is to allow:

@cbv_flow = if session[:cbv_flow_id]
              CbvFlow.find(session[:cbv_flow_id])
            end

[tdooner]

404 after having started a CBV flow:

404 in a new window/incognito:

500:

[tdooner]

We previously had this config to exclude /cbv/employer_search from our CSP checks. But it doesn't seem necessary anymore (or else the owasp-zap action would have failed after I removed it.)

[tdooner]

Sam acceptance-tested and approved via Slack!