Sends the real error page from a controller or a plug.

render_errors only catches errors that reach the endpoint as an exception — a route that never matched, or something that raised. Code which decides for itself that a request is a 404 and answers with text("Not Found") never goes near it, so a reader got nine bytes of plain text while the error template sat unused. That is what /ads did.

Rendering to a string rather than going through put_view keeps this usable from a plug, where there is no view or layout to configure, and guarantees the page is byte-identical to the one render_errors produces.