check repo icons exist now before running through all of `fdroid update`

check repo icons exist now before running through all of `fdroid update`

Before, if repo_icon or archive_icon pointed to a non-existent file, then
`fdroid update` would run through the whole process of building a repo,
then fail at the very end because of the non-existent file.  On the next
run, `fdroid update` then starts from the beginning.

This just checks for those files at the beginning, and exits with an error
if they are not found.