In certain scenarios (such as when quota is exceeded), it's possible for: 1. A metadata entry to exist (i.e. `checkCachedPackage()` calls its success callback), 2. But the package data to be missing (so `fetchCachedPackage()`'s `onsuccess` handler receives an `event.target.result` that's undefined) **Expected behavior:** `fetchCachedPackage()`'s `errback` gets called, so that the `preloadFallback` gets triggered **Current behavior:** An unhandled exception gets thrown **Version of emscripten/emsdk:** [3.1.53](https://github.com/emscripten-core/emscripten/releases/tag/3.1.53)