BUG/MINOR: resolvers: new callback to properly handle SRV record errors

BUG/MINOR: resolvers: new callback to properly handle SRV record errors

When a SRV record was created, it used to register the regular server name
resolution callbacks. That said, SRV records and regular server name
resolution don't work the same way, furthermore on error management.

This patch introduces a new call back to manage DNS errors related to
the SRV queries.

this fixes github issue #50.

Backport status: 2.3, 2.2, 2.1, 2.0

(cherry picked from commit b4badf720ce484001f606011aee7cd216e5ce4e3)
[cf: Changes applied in src/dns.c and structures renamed]
Signed-off-by: Christopher Faulet <cfaulet@haproxy.com>