http-push: using error() and warning() as appropriate

Change three occurences of using inconsistent error/warning reporting by
using the relevant error() / warning() calls to be consitent with the
rest of the code.

Signed-off-by: Miklos Vajna <vmiklos@frugalware.org>
Signed-off-by: Junio C Hamano <gitster@pobox.com>