http: enable keep_error for HTTP requests

http: enable keep_error for HTTP requests

curl stops parsing a response when it sees a bad HTTP status code and it
has CURLOPT_FAILONERROR set. This prevents GIT_CURL_VERBOSE to show HTTP
headers on error.

keep_error is an option to receive the HTTP response body for those
error responses. By enabling this option, curl will process the HTTP
response headers, and they're shown if GIT_CURL_VERBOSE is set.

Signed-off-by: Masaya Suzuki <masayasuzuki@google.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>