Handle attempted download of unavailable module zips better

When a module is listed in mods.d.tar.gz but the zip is missing the subsequent '404 Not Found' html web page is downloaded into a zip file. The first error seen relates to the invalid downloaded zip rather than the missing module.

So we need to check the zip is actually available before doing the download. A module being listed in the mods.d.tar.gz file does not guarantee it is available e.g. Crosswire AV repo.

I have attached a sample zip that was downloaded for a missing module. If you open it in NotePad (despite it being a zip) you will see the error message.