If response is 404, skip trying other CDNs

When there is a 404 response, in all likelihood other
CDNs will also return 404, no point pointlessly fetching
from others.

src/js/diff-updater.js

@@ -194,6 +194,7 @@ async function fetchPatchDetailsFromCDNs(assetDetails) {
             console.error(reason);
         });
         if ( response === undefined ) { continue; }
+        if ( response.status === 404 ) { break; }
         if ( response.ok !== true ) { continue; }
         const patchText = await response.text();
         const patchDetails = parsePatch(patchText);