Safely notify users when an incorrect zip is uploaded.

Hello,

Many of my projects are delivered in an overall zip file which contains help files, PSDs and so on in addition to the actual theme zip. If they upload the entire package, it of course fails since it’s not meant to be uploaded.

Is there a clean way to include something in the zip root that would stop the theme activation process and simply kick out an error message I could customize? Currently, WordPress outputs the stylesheet missing or failed to open stream error on failed theme uploads.

Thanks!