Cache SW: Ensure any failure is always logged. (ampproject#8328)

* Ensure any failure is always logged.

* lint

* Tell closure compiler it'll be alright.

src/service-worker/core.js

@@ -597,7 +597,7 @@ export function handleFetch(request, maybeClientId) {
 
   // Wait for the cachePromise to resolve. This is necessary
   // since the SW thread may be killed and restarted at any time.
-  return cachePromise.then(() => {
+  return /** @type {!Promise<!Response>} */ (cachePromise.then(() => {
     // If we already registered this client, we must always use the same
     // version.
     if (clientsVersion[clientId]) {
@@ -625,7 +625,11 @@ export function handleFetch(request, maybeClientId) {
       // If not, let's fetch and cache the request.
       return fetchJsFile(cache, versionedRequest, version, pathname);
     });
-  });
+  }).catch(err => {
+    // Throw error out of band.
+    Promise.reject(err);
+    throw err;
+  }));
 }