sogrep: fail if links database cannot be retrieved
If the links database (for some reason) does not exist on the mirror, curl will save the html 404 page as ${repo}.links.tar.gz in the cache, and sogrep will later fail with a decompression error from bsdtar.
This patch adds -f to curl so it doesn't save the error page, and exit after displaying an error in such case.
(originally sent to the mailing list but