mod-blocks preview: a HEAD 404 skips the build-time framing check #2287

The preview block's build-time framing check (mod-blocks v2.13.0, layouts/partials/assets/preview.html) retries a HEAD request with GET only when $response.Err is set. Hugo's resources.GetRemote returns no resource and no error for a 404, so a target that answers HEAD with 404 is never re-checked with GET. Hugo then warns Request to '<url>' failed: <nil>. Rendering the preview anyway. and renders the block live, even when GET would return X-Frame-Options: DENY or a restrictive CSP frame-ancestors.

Reproduction: run a local server that answers 404 to HEAD, and 200 with X-Frame-Options: DENY to GET. Allow the server in the site's frame-src and render a preview of it. The server logs only the HEAD request. The iframe has no d-none.

Impact: low. At runtime, the readiness/timeout fallback still covers a frame that the browser refuses. The build-time fallback and its warning are skipped, and the warning text "failed: " is misleading.

Possible direction: also retry with GET when HEAD yields no resource (not $response.Value), or when it yields a non-2xx status.

Found by an independent review of the mod-docs preview documentation (#2268, Amfelt homepage live-demo program). Filed separately; this is not a change to the released v2.13.0 scope. The docs now describe the current behavior.