Handle missing zip assets non-catastrophically

Combined with previous commit, this hopefully fixes #993

Author: GarboMuffin

src/packager/packager.js

@@ -1121,7 +1121,8 @@ cd "$(dirname "$0")"
             const path = assetId + '.' + dataFormat;
             const file = findFileInZip(path);
             if (!file) {
-              throw new Error('Asset is not in zip: ' + path)
+              console.error('Asset is not in zip: ' + path);
+              return Promise.resolve(null);
             }
             return file
               .async('uint8array')