Update rabbit_federation_exchange_link.erl

=ERROR REPORT==== 7-Mar-2014::12:49:24 ===
connection <0.1484.227>, channel 2 - soft error:
{amqp_error,not_found,
            "no exchange 'federation: amq.fanout -> rabbit@baitweb-bd0001-lnx.qualcomm.com A' in vhost '/'",
            'exchange.delete'}

I saw many ERROR of above one, I think rabbit_federation_exchange_link:delete_upstream_exchange is the contributor. I don't think it's necessary to log the federation exchange delete error. Instead, we might can add a exchange_exist check before call it?