Fail properly when cloning from invalid HTTP URL

Fail properly when cloning from invalid HTTP URL

Currently, when cloning from invalid HTTP URL, git clone will possibly
return curl error, then a confusing message about remote HEAD and then
return success and leave an empty repository behind, confusing either
the end-user or the automated service calling it (think repo.or.cz).

This patch changes the error() calls in get_refs_via_curl() to die()s,
akin to the other get_refs_*() functions.

Cc: Daniel Barkalow <barkalow@iabervon.org>
Signed-off-by: Petr Baudis <pasky@suse.cz>
Signed-off-by: Junio C Hamano <gitster@pobox.com>