http: avoid empty error messages for some curl errors

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>