I was just walking along and suddenly *POOF* - "Failed: Bring one Black Briar Mead to Siddgeir." Is there a time limit on quests? Or did he get killed maybe?

I don't think people can die (or do anything, really) if you're not in the same cell as them, so I don't think he died.

But I think if you won the civil war as a Stormcloak he doesn't have the same dialogue options available since he sided with Imperials, maybe? Not sure...