Minor: Clear error message in case of successful DL but if not a valid ZIP

This is useful to debug problems e.g. when using Dropbox to host Mod Packs and when forgetting the &dl=1 parameter Dropbox needs to serve the ZIP instead of its HTML page. Before this micro enhancement, one (me..) may think that the DL didn't work, when it actually did - it just didn't download a ZIP (as the codes checks). With this additional log, it's clearer to the end-user why a (non-ZIP) "mod pack was not downloaded".

Unrelated: Removed no longer needed @SuppressWarnings("unused")