file_packager.py: Handle failure to get cached package

In certain scenarios (such as when quota is exceeded), it's possible for a metadata entry to exist, but the package data to be missing - and `event.target.result` will be undefined.

Check for and properly handle this by calling `errback`, so that the `preloadFallback` gets triggered.

tools/file_packager.py

@@ -914,6 +914,10 @@ def generate_js(data_target, data_files, metadata):
           for (var chunkId = 0; chunkId < chunkCount; chunkId++) {
             var getRequest = packages.get(`package/${packageName}/${chunkId}`);
             getRequest.onsuccess = function(event) {
+              if (!event.target.result) {
+                errback(new Error("CachedPackageNotFound for: " + packageName));
+                return;
+              }
               // If there's only 1 chunk, there's nothing to concatenate it with so we can just return it now
               if (chunkCount == 1) {
                 callback(event.target.result);