http: set curl FAILONERROR each time we select a handle

Because we reuse curl handles for multiple requests, the
setup of a handle happens in two stages: stable, global
setup and per-request setup. The lifecycle of a handle is
something like:

  1. get_curl_handle; do basic global setup that will last
     through the whole program (e.g., setting the user
     agent, ssl options, etc)

  2. get_active_slot; set up a per-request baseline (e.g.,
     clearing the read/write functions, making it a GET
     request, etc)

  3. perform the request with curl_*_perform functions

  4. goto step 2 to perform another request

Breaking it down this way means we can avoid doing global
setup from step (1) repeatedly, but we still finish step (2)
with a predictable baseline setup that callers can rely on.

Until commit 6d052d7 (http: add HTTP_KEEP_ERROR option,
2013-04-05), setting curl's FAILONERROR option was a global
setup; we never changed it. However, 6d052d7 introduced an
option where some requests might turn off FAILONERROR. Later
requests using the same handle would have the option
unexpectedly turned off, which meant they would not notice
http failures at all.

This could easily be seen in the test-suite for the
"half-auth" cases of t5541 and t5551. The initial requests
turned off FAILONERROR, which meant it was erroneously off
for the rpc POST. That worked fine for a successful request,
but meant that we failed to react properly to the HTTP 401
(instead, we treated whatever the server handed us as a
successful message body).

The solution is simple: now that FAILONERROR is a
per-request setting, we move it to get_active_slot to make
sure it is reset for each request.

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