http-push: improve error log

http-push: improve error log

When git push fails due to server-side WebDAV error, it's not easy to
point to the main culprit.  Additional information about exact cURL
error and HTTP server response is helpful for debugging purpose.

New error log helped me pinpoint failing test t5540-http-push-webdav
to a missing Apache dependency in Fedora 27:
https://bugzilla.redhat.com/show_bug.cgi?id=1491151

Signed-off-by: Patryk Obara <patryk.obara@gmail.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>