error: add global &error_warn destination

This can help debugging issues or develop, when error handling is
introduced.

Signed-off-by: Marc-André Lureau <marcandre.lureau@redhat.com>
Reviewed-by: Stefan Berger <stefanb@linux.ibm.com>
Message-Id: <20230221124802.4103554-6-marcandre.lureau@redhat.com>