make it possible for fedmsg_tail_messages to fail if it cannot connect to fedmsg

[tyll]

autosign01 used hub.fedoraproject.org as endpoint, but at least one IP that is returned for it is not reachable from autosign01, therefore fedmsg.tail_mesages should be able to raise exceptions if it cannot reach the endpoint or re-try after performing a new DNS lookup. Or handle thos somehow differently.