iframe-proxy: inject shim on error pages

Author: nedtwigg

docs/specs/dor-iframe.md

@@ -157,7 +157,9 @@ precise error *page* from the proxy origin (which frames fine): a refused remote
 responding at `localhost:8080` — is the dev server running?". `createIframeProxyUrl`
 itself returns `{ ok: false, reason }` only for the synchronous cases (chiefly an
 unproxyable `scheme`); reachability and frame-refusal are surfaced as served
-pages.
+pages. These served error pages include the same fixed leader shim as proxied
+HTML, so the keyboard escape path still works after the user clicks inside an
+error state.
 
 ### Cursor alignment (the out-of-process-frame offset)
 

vscode-ext/src/iframe-proxy-host.ts

@@ -375,7 +375,7 @@ function unreachablePage(upstream: URL, detail: string): ErrorPage {
 }
 
 function serveErrorPage(res: http.ServerResponse, page: ErrorPage): void {
-  const html = errorPageHtml(page);
+  const html = instrumentHtml(errorPageHtml(page));
   res.writeHead(200, {
     'content-type': 'text/html; charset=utf-8',
     'content-length': Buffer.byteLength(html).toString(),