When proxying, instead of rendering the dashboard with an error page we should render a small non-SPA HTML file that contains the error message and a button to refresh, a button to go to dashboard (controlled by Go templates).

We have this in v1 for when you get proxy errors. It can be reproduced by creating a dev url and trying to connect to it without starting an app on that port first:

Requirements

1. Completely self-contained page, with no<link rel="stylesheet" ...>s or<script src="..."/>s
2. Go template variables for:
   • status codeError.Status
   • error titleError.Title
   • error descriptionError.Description
   • optional button to reload based onError.Reloadable
   • button to redirect to dashboardError.DashboardURL
3. Stored atsite/static/simple-error.html or something like that