clarifying comment in alt-fetch plugin code

plugins/alt-fetch/index.js

@@ -105,6 +105,10 @@
                 ))
                 .then((response) => {
                     // 4xx? 5xx? that's a paddlin'
+                    // NOTICE: normally 4xx errors are returned to the client by other plugins,
+                    // NOTICE: but here we are relying on multiple alternative endpoints;
+                    // NOTICE: so, we want to maximize the chance that we get *something* useful
+                    // TODO: shouldn't this reject() instead
                     if (response.status >= 400) {
                         // throw an Error to fall back to other plugins:
                         throw new Error('HTTP Error: ' + response.status + ' ' + response.statusText);