store: move TempZipFromFiles helper to only call site

Created by: keegancsmith

This helper was reusing the store to create a zip file on disk. This is a bit of overkill, so this commit simplifies the function. Additionally there is only one call site for this helper, so we can reduce the amount of exported API.

I also noticed one latent issue where we ignored the errors returned by the underlying functions we are testing. We now check those errors.