Update 404/500 to split out actions for form builder and form filler (Update 2)

[anikbrazeau]

Description

Different CTAs that are appropriate to each audience when these pages 404/500 are encountered, whether from the app or from a public-facing form

Problem: End users filling out a form (form fillers) get a page that redirects them to Support.

Context

Follow up to #4502 which covered generic errors for basic form elements.

User story

As an end-user, I need more specific next steps when something goes wrong, so that I can navigate back to the right place or know what to do next.

Additional detail

See rows: 17-22
https://docs.google.com/spreadsheets/d/1qx6K-PkburkQ2anDbRAhLpgWIfbYF8aWV6N34NrFUp8/edit?gid=0#gid=0

Acceptance criteria

• Includes English and French error message content
• Right message is displayed to right audience (segmenting builders and fillers)
• Displays properly in Chrome, Firefox, Edge, Safari, and Opera
• Reads in the expected order on screen reader
• Works when there are multiple similar errors triggered on a page
• Provides context for the error focus state at the top of the error message list
• Links to the correct error / question
• Meets WCAG 2.1 AA requirements