dynamicproxy/tools: set up outage error system

This produces a prod-like error message.

Configured using Hiera:
"dynamicproxy::error_enabled": true
"dynamicproxy::error_message": "Our servers are experiencing technical issues ...etc"
"dynamicproxy::error_details": "..."

Bug: T102971
Change-Id: Ie040cb539cc402b526a18216595f75e9505dff12