setup-standalone-server.sh leaves empty folders for missing packages

[dri-richard]

If a package is missing from DM_PACKAGES_URL, download_package() will leave an empty folder in extender/platformsdk/ when running rebuild-and-run-standalone-local.sh

This has two consequences:

1. A visual inspection of extender/platformsdk/ in Finder suggests the item has been copied because the SDK folder is present
2. If the packaged sdk is added to the DM_PACKAGES_URL location and rebuild-and-run-standalone-local.sh is run again, the folder is detected and the download is skipped because it appears to already be installed.