http: avoid empty error messages for some curl errors
When asked to fetch over SSL without a valid
/etc/ssl/certs/ca-certificates.crt file, "git fetch" writes
error: while accessing https://github.com/torvalds/linux.git/info/refs
which is a little disconcerting. Better to fall back to
curl_easy_strerror(result) when the error string is empty, like the
curl utility does:
error: Problem with the SSL CA cert (path? access rights?) while
accessing https://github.com/torvalds/linux.git/info/refs
Signed-off-by: Jonathan Nieder <jrnieder@gmail.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>