feat: add custom error page for 500 errors

[stephenliang]

A new sad bee page was created specifically for 500s to instruct the user to sign out, clear cookies, and run other troubleshooting steps that most commonly resolve 500 errors. However, this page does not apply to other error codes (503, 504, 404, etc.) as we don't necessarily want users to be signing out when the server is timing out or having other issues.

Testing story

Created adhoc with adhoc:cdn:start

Updated config/adhoc.yml.erb and set custom_error_response to true. Observed updated distribution error page configuration

Deployment strategy

Follow-up work

Privacy

Security

Caching

PR Checklist:

• Tests provide adequate coverage
• Privacy and Security impacts have been assessed
• Code is well-commented
• New features are translatable or updates will not break translations
• Relevant documentation has been added or updated
• User impact is well-understood and desirable
• Pull Request is labeled appropriately
• Follow-up work items (including potential tech debt) are tracked and linked

[Hamms]

LGTM; thanks!