HACKING: Add a section on error handling and reporting

Inspired by an RFC PATCH from Lluís Vilanova.

Signed-off-by: Markus Armbruster <armbru@redhat.com>
Message-Id: <1454522628-28294-3-git-send-email-armbru@redhat.com>
Reviewed-by: Lluís Vilanova <vilanova@ac.upc.edu>
