http: style fixes for curl_multi_init error check

Unless there is a good reason, we should use die() rather than
fprintf/exit. We can also shorten the message to match other curl init
failures (and match our usual lowercase no-full-stop style).

Signed-off-by: Jeff King <peff@peff.net>
Signed-off-by: Junio C Hamano <gitster@pobox.com>